Praegnatio Perturbatio—Impact of Endocrine-Disrupting Chemicals

Abstract

The burden of adverse pregnancy outcomes such as preterm birth and low birth weight is considerable across the world. Several risk factors for adverse pregnancy outcomes have been identified. One risk factor for adverse pregnancy outcomes receiving considerable attention in recent years is gestational exposure to endocrine-disrupting chemicals (EDCs). Humans are exposed to a multitude of environmental chemicals with known endocrine-disrupting properties, and evidence suggests exposure to these EDCs have the potential to disrupt the maternal-fetal environment culminating in adverse pregnancy and birth outcomes. This review addresses the impact of maternal and fetal exposure to environmental EDCs of natural and man-made chemicals in disrupting the maternal-fetal milieu in human leading to adverse pregnancy and birth outcomes—a risk factor for adult-onset noncommunicable diseases, the role lifestyle and environmental factors play in mitigating or amplifying the effects of EDCs, the underlying mechanisms and mediators involved, and the research directions on which to focus future investigations to help alleviate the adverse effects of EDC exposure.

Graphical Abstract

Graphical Abstract.

ESSENTIAL POINTS.

1. One of the major risk factor affecting adverse birth outcomes is gestational exposure to endocrine-disrupting chemicals (EDCs) with disruptions manifested at the level of maternal, fetal/neonatal and placental milieu.

2. Lifestyle factors such as diet and stress interact with EDCs to mitigate or amplify the effects of EDCs.

3. EDC action involves mediators such as inflammation, oxidative stress, hormonal and metabolomic changes, microbiome and epigenetic alterations.

4. Risk assessments would require consideration of EDC exposure burden in relation to site, type and matrix of EDC measurement, dose-response relationships, susceptibility windows, fetal/neonatal sex, lifestyle factors, placental contribution, and analytical approaches employed for appraising composite risk and target interventions for mitigating the adverse effects of EDC exposure.

The development of any organism into an adult depends on an intricate interaction of processes that regulate cell proliferation and differentiation along with integration of numerous factors that include genetics and environment. These complex processes need to be coordinated over various developmental windows that span prenatal and postnatal life. Organismal developmental plasticity, by which a single genotype can give rise to a range of different physiologic or morphologic states in response to prevailing environmental conditions, characterizes early periods of development (1). By adolescence, individuals lose this plasticity and mature into an adult phenotype of physiological capacity and morphology. Developmental plasticity during early life comprises successive or parallel windows of susceptibility of organ systems to environmental insults. Developmental adaptations made in the short term to early-life insults ensure immediate survival by selectively reducing functional capacity of some organs so as to channel resources to critical organs (2). Epidemiological and experimental data, however, indicate that such adaptive responses can prove to be maladaptive in the long term, compromising health (3). For example, babies born during the 1944 to 1945 Dutch famine, who faced malnutrition during intrauterine life, had increased risk of many chronic cardiovascular and metabolic diseases (4-6). The long-term health outcomes resulting from developmental impact were formulated by David Barker as the fetal origin of disease theory, which is now formalized as the developmental origin of health and disease (DOHaD) hypothesis (7). Since its formulation, epidemiological and animal experimentation studies have provided a large body of evidence confirming the validity of this hypothesis.

Developmental insults leading to maladaptive responses range from maternal disease, nutritional deficit/excess, stress states, and exposure to environmental factors that include lifestyle choices and chemical and climate exposure (8). Of these, the impact of early exposure to environmental chemicals capable of disrupting the endocrine system, referred to as endocrine-disrupting chemicals (EDCs), has gained prominence in recent years. This review focuses on (a) the early impacts of environmental EDCs in modulating the maternal-fetal environment, predominantly in humans (barring a few examples from experimental animal models relative to causality) resulting in adaptive changes in the fetus and adverse pregnancy outcomes; (b) the mechanisms and intermediaries involved; (c) the long-term health consequences in brief (an outcome extensively reviewed previously [8-10]); (d) the role that genetic makeup, lifestyle, and environmental factors play in modifying the effects of EDCs, and (e) research directions on which to focus future investigations that would aid in the development of intervention strategies to mitigate the adverse developmental effects of EDCs.

Endocrine-Disrupting Chemicals

Hormones are major factors in the maternal and fetal milieu that influence the developmental trajectory of the offspring in a dose-, time-, and organ-specific manner. Any agent that acts as an agonist or antagonist of endogenous hormones thus has the potential to influence developmental trajectory of the offspring. In this context, EDCs as defined by the US Environmental Protection Agency (EPA) are “exogenous agents that interfere with synthesis, secretion, transport, metabolism, binding action, or elimination of natural blood-borne hormones that are present in the body and are responsible for homeostasis, reproduction, and developmental process” (11). Through their action EDCs have the potential to disrupt normal endocrine functions and disturb the maternal and fetal endocrine milieus (12, 13). Although certain chemicals were known for their hormone-disrupting function by the mid-20^th century, the term endocrine disrupter was first used in the 1991 Wingspread meeting (14). Initial research into EDCs focused mostly on chemicals with steroidal capacity and has now expanded to include chemicals that span a wide range of hormonal functions.

The Toxic Substances Control Act mandates the US EPA maintain a list of chemicals that are manufactured, processed, or imported into the United States that includes about 86 000 chemical substances. A 2012 estimate by the World Health Organization indicated about 800 chemicals, used in everyday life, to be EDCs (15). In 2020, the Endocrine Disruption Exchange database (https://endocrinedisruption.org/interactive-tools/tedx-list-of-potential-endocrine-disruptors/search-the-tedx-list) listed 1482 chemicals with endocrine-disrupting potential highlighting the increase in number of chemicals recognized as EDCs. Broadly speaking, EDCs include both natural and anthropogenic chemicals that humans are exposed to in everyday life (Fig. 1) (13, 16). Naturally occurring compounds with endocrine-disrupting potential include metals and metalloids, parabens, polyaromatic hydrocarbons (PAHs), and phytoestrogens. Man-made synthetic chemicals are commonly used in agricultural practices (pesticides, insecticides, and fungicides), packaging (food-storage materials, plastics), industry (solvents, flame retardants, preservatives, emulsifiers, fracking chemicals), consumer products (household chemicals, cosmetics, flame retardants, building materials, children’s toys, electronics, cookware), and medical care (birth control pills, biocides, intravenous bags and tubing, disposable gloves, disinfectants). Although chemicals such as metals and PAHs exist naturally, increase in their exposure occur as a result of human activity such as mining for metals and burning fossil fuel.

Figure 1.

Schematic showing the different classes of endocrine-disrupting chemicals (EDCs) that pregnant women are commonly exposed to during everyday life, their mechanism of endocrine disruption and mediators involved in influencing maternal, placental, and fetal milieu. AhR, arylhydrocarbon receptor; AR, androgen receptor; ESR, estrogen receptor; GR, glucocorticoid receptor; PGR, progesterone receptor; PPAR, peroxisome proliferator activated receptor; TR, thyroid receptor.

Relevant to the focus of this review, EDCs are detectable in the maternal-placental-fetal unit during different stages of gestation and at delivery (Table 1). Several of these EDCs were detected in pregnant women as evidenced from the National Health and Nutrition Examination Survey (NHANES) in the United States and the Maternal-Infant Research on Environment Chemicals study in Canada (17, 18). About 40 to 50 chemicals have been measured in women during pregnancy (17, 19), highlighting the potential risk to maternal and fetal health from such exposures. Emerging epidemiological studies, both prospective and retrospective, have documented adverse effects of EDCs that range from poor gestational/early birth outcomes (13, 19, 20) to long-term adverse health effects in the offspring (13, 21-24). Exposure to EDCs also come at increased economic consequences although specific estimates relative to exposure during pregnancy are not available. Conservative estimates from 2016 based on the attributable fraction of a risk factor that could decrease the number of cases of disease or deaths by reducing the risk factor places disease costs of EDC exposures at about $340 billion in the United States and €163 billion in Europe (25, 26). The majority of the cost in the United States appears to be associated with polybrominated diphenyl ether (PBDE) exposure ($266 billion), whereas in the European Union organophosphate pesticides seem to be the largest contributors (€146 billion) (25, 26).

Table 1.

Common endocrine-disrupting chemicals with their known role in endocrine disruption and detection in humans

EDC class	Example EDC	Endocrine disruption	EDC presence detected in
			Maternal urine	Maternal blood	Placenta	Umbilical cord blood	Amniotic fluid	Fetal tissue
Naturally occurring EDCs
Metals	Cd Hg Ar Cr	Pb, Cd, and Hg: placental hormone biosynthesis (27, 35) Ar: steroid receptor function (27)	TM1 heavy metals: As, Cd, Cr, Pb, Tl, V (19, 36–38) As (39) Cd (39-41) TM2 As (38, 42-44) Cd (40) TM3/Term U (45-47) As (38, 39, 41, 48, 49) Cd (40) Ni (50)	TM1 Heavy metals (37, 39) TM3/Term Heavy metals (34, 39, 51, 52) Hg (53) Cd (54)	Term Heavy metals (32-34, 52) Cr (35) Pb (55, 56)	Term Heavy metal (34, 39, 52, 57, 58) Hg (53, 59)	Unspecified Heavy metals (60) Term Heavy metals (51)
Parabens	Methylparaben Propylparaben Butylparaben Ethylparaben	Estrogenic and antiandrogenic actions (66) progesterone receptor, thyroid receptor and PPAR (67)	TM1 (19) TM3/Term (63, 68)	TM1 (68) TM2 (69)	TM3/Term (64, 68)	Term (68)	Term (68)
PAHs	Benzo(a) pyrene benzo(a) anthracene benzo(b) fluranthene indeno(1,2,3-cd) pyrene naphthalene	Aryl hydrocarbon receptor binding (75, 76); inhibit estrogen receptor action (80)	Unspecified (17) TM3/Term (77, 78)	TM2 (79) TM3/Term (73)	TM3/Term (72, 73, 81)	TM3/Term (73)	TM3/Term (74)	TM2 (79)
Phytoestrogens	Lignans, isoflavones (daidzein, genistein, and glycitein) coumestans (coumestrol)	Estrogen receptor agonists (86)	TM3/Term (84)	TM3/Term (84, 85)		Term (83-85, 88)	TM2 (89, 90) Term (85, 91)
EDCs in plastics
Bisphenols	BPA, BPB, BPC, BPE, BPF, BPS, BPZ, BPAF, and BPAP	Estrogenic activity (101); Anti-androgenic activity and modulation of glucocorticoid, PPAR and thyroid systems (102-104)	TM1 (19, 40) TM3/Term (40)	TM1 (20) (20, 69) TM2 (43, 69) TM3/Term (20, 50, 105, 106)	Term (94, 105)	Unspecified (107) TM2 (108) Term (20, 50, 105, 106)	TM2 (98, 99) TM3 (98)	Unspecified (107) TM1-TM2 (100) TM2 (108)
Phthalates	DEHP MEHP DBP DMP	Estrogenic, anti-estrogenic, anti-androgenic and metabolic actions (115-117)	Unspecified (17) TM1 (19, 40, 110) TM2 (112) TM3/Term (40, 110-112)	TM2 (43) TM3/Term (112-114)		Term (112-114)	Unspecified (118) TM2 (119)
Industrial chemicals
Organo-halogens	PCBs PBDEs TBBPA HBCDD PFAS (PFOS and PFOA)	PCB and PBDE: disruption of estrogen, androgen or thyroid signaling (129) PFAS: estrogenic activity (123) Halogenated bisphenol: estrogenic and PPAR action (130)	Unspecified (17)	TM1 PCB and PBDE (126) TM2 PCB and PBDE (43, 127) PFAS (43, 125) TM3/Term PFAS (113) PBDE and PFOS (50)	TM2 PFAS (125)	TM2 PCB and PBDE (127) TM3/Term PFAS (113) PBDE and PFOS (50) PCB and PBDE (126)	TM1 PFAS (124, 125) TM2 PCB and PBDE (127) PFAS (124, 125) Term PFAS (125)	TM1 PFAS (124, 125) TM2 PFAS (124, 125) PCB and PBDE (127) Term PFAS (125)
Agrochemicals
Pesticides	OC, OP, carbamates	Organochlorines: interference of AHR action (136) organophosphates and carbamates: inactivation of acetylcholinesterase enzyme (141)	TM1 DDE (143) TM2 DDE (143)	TM3/Term DDE (122, 159, 160) OP (50)	TM3/Term DDE (122) DDT and HCH (81)	TM3/Term DDE (122, 159, 160) OP (50)
Herbicides	Atrazine, simazine, and propazine	Increases aromatase expression (146)	TM1 (143) TM2 (143, 161, 162) TM3 (162)
Fungicide	Vinclozolin	Anti-androgenic effects (149, 150)				TM3/Term (148)
Antifouling agents	Tributyltin, Chlorothalonil, Dichlofluanid, Diuron	Agonistic ligand for RXR and PPAR γ (156) Placental steroidogenesis and chorionic gonadotropin secretion (157)			TM3/Term (155, 163)
Anti-bacterials	Triclosan triclocarban	Interferes with estrogen, androgen, and thyroid hormone action (174, 175)	TM1 (19, 176, 177) TM3/Term (171)	TM1 (69) TM2 (69) TM3/Term (172, 173)		TM3/Term (173)	TM3/Term (172)
Medical products
Pharma-ceuticals	Diethylstilbestrol	Estrogen receptor agonist anti-androgenic action (180-184)	Unspecified (179)
Medical Supplies (Bisphenols and Phthalates)	see above sections for detection of bisphenols and phthalates

Abbreviations: AHR, arylhydrocarbon receptor; EDC, endocrine-disrupting chemical; HBCDD, hexabromocyclododecane; HCH, hexachlorocyclohexane; MDA, malondialdehyde; MEHP, mono(2-ethylhexyl) phthalate; mid to late, second and third trimester; mRNA, messenger RNA; miRNA, microRNA; OC, organochlorines; OP, organophosphorus; PAHs, polyaromatic hydrocarbons; PBDE, polybrominated diphenyl ether; PCB, polychlorinated biphenyl; PFAS, perfluorinated alkylated substance; PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonic acid; PPAR, peroxisome proliferator activated receptor; RXR, retinoid X receptor; T3, 3,5,3′-triiodothyronine; T4, thyroxine; TBBPA, tetrabromobisphenol A; TM, trimester; TNF, tumor necrosis factor; TSH, thyrotropin.

Naturally Occurring Endocrine-Disrupting Chemicals

This section focuses on naturally occurring products with proven endocrine-disrupting activity that humans are exposed to through diet or inhalation.

Metals

Certain metals called trace metals are indispensable for living organisms and are sourced through food—these metals when in excess can be detrimental. Other metals although found naturally can be toxic when released into the environment in large quantities by industrial activities and enter humans and animals along the food chain, leading to higher levels of exposure and serious health risks. Most studies assessing the endocrine-disrupting potential of metals have focused on cadmium (Cd), mercury (Hg), arsenic (As), and lead (Pb). In recent years’ other metals, especially chromium (Cr), manganese (Mn) and zinc (Zn) are also being acknowledged as EDCs (27). Inappropriate exposure to most metals is through food and water contamination that occurs mostly from pollution from industrial activity (28-30). Consumer exposures to these metals can also occur from their presence in tobacco smoke (Cd), paints and batteries (Cd, As, Hg, and Pb), and dental amalgam (Hg) (28-30). The 1999 to 2016 NHANES reported detection rates of 83 to 99% based on urinary levels of As, Pb, Hg and Cd both in pregnant and nonpregnant women of child bearing age. Importantly, the majority of these metal concentrations were found to be higher in pregnant compared to nonpregnant women, the result of usage of multivitamin and multimineral supplements or dietary changes (31). These metals have also been found in placenta, newborn cord blood, and amniotic fluids (see Table 1) (27, 32-34), indicative of placental transfer. A wide range of endocrine disruptions is attributed to metals such as disruptions of placental progesterone synthesis through inhibition of cytochrome P450 enzymes by Cd and Hg and steroidal hormone receptor action such as estrogen androgen and glucocorticoid receptors by As (27, 35).

Parabens

Parabens are aliphatic or aromatic alkyl esters of p-hydroxybenzoic acid, which are naturally present in some fruits and vegetables. They are of environmental concern because of their widespread use as preservatives in cosmetics, drugs, and foods (61). Although categorized as “generally recognized as safe” by the US Food and Drug Administration, emerging data suggest adverse effects of parabens, particularly during early developmental stages (61, 62). The most commonly used parabens are methylparaben, propylparaben, butylparaben, and ethylparaben. Parabens have been detected in maternal urine (19, 63). A significant correlation exists between maternal urinary paraben concentrations and their newborn infant levels, supporting transfer of parabens from mother to fetus (63). Parabens have also been detected in human placental tissue (64) and amniotic fluid (65) (see Table 1). Parabens have both estrogenic and antiandrogenic actions as the basis for their endocrine disruptions (66) but are also known to influence the progesterone receptor, thyroid receptor and peroxisome proliferator-activated receptor (PPAR) (67).

Polycyclic aromatic hydrocarbons

PAHs are naturally occurring organic compounds in coal, crude oil, and gasoline that result from incomplete combustion or pyrolysis of organic matter. These are mainly generated by combustion processes, such as wood burning, motor vehicle exhaust, cooking and agricultural waste burning (70). Cigarette smoking is another source of exposure to many PAHs (71). As part of the atmospheric pollutants they include compounds like benzo(a) pyrene, benzo(a) anthracene, benzo(b) fluoranthene, benzo(k) fluoranthene, chrysene, dibenzo(a, h) anthracene, indeno(1,2,3-cd) pyrene and naphthalene, also classified as carcinogens by the EPA. The 2003 to 2004 NHANES detected urinary PAHs in approximately 99% to 100% of US pregnant women (17). PAHs and PAH-DNA adducts have been detected in maternal blood, cord blood and placentae (72, 73), amniotic fluid (74) and fetal tissues (79) (see Table 1). The effects of PAHs in disrupting endocrine functions are mediated through their ability to bind aryl hydrocarbon receptor (AHR) (75, 76) but also have been shown to involve estrogen receptor action (80).

Phytoestrogens

Phytoestrogens, naturally occurring plant compounds, are known for their weak estrogenic effects (82). The presence of soy, a major source of phytoestrogen in many baby formulas, is of particular developmental concern. Phytoestrogens are categorized as lignan, isoflavones (daidzein, genistein, and glycitein), and coumestans (coumestrol). Phytoestrogens can be transferred from mother to fetus (83) and have been detected in maternal blood, maternal urine, cord blood, and amniotic fluid (84, 85) (see Table 1). The main endocrine-disrupting role of phytoestrogens involves their binding to estrogen receptors, with coumestrol having the highest affinity, followed by genistein (86). Phytoestrogens have also been found to have inhibitory effects on placental growth factor and chorionic gonadotropin biosynthesis (86, 87).

Anthropogenic Chemicals

This section specifically deals with man-made synthetic chemicals, the environmental burden of which has significantly increased with the industrial revolution and urbanization.

Endocrine-disrupting chemicals in plastics

Bisphenols are industrial chemicals used in the manufacture of plastics. Bisphenol A (BPA), which has been in use since 1960, is found in certain plastics and resins such as polycarbonate plastics (eg, water bottles) and epoxy resins used to coat the inside of metal products (food cans, bottle tops, and water supply lines) (92). It is also present in thermal paper used for cash receipts and some dental sealants and composites (92). More recently analogues of BPA such as BPB, BPC, BPE, BPF, BPS, BPZ, BPAF, and BPAP have been used as BPA replacements (93). Owing to their widespread usage, BPA and its analogues have been detected in 75% of food samples tested (91) and are regularly detected in human urine and blood (94, 95). According to the 2013 to 2014 US NHANES, 96%, 88%, and 66% of the women of reproductive age (15-44 years) had urinary concentrations of BPA, BPS, and BPF, respectively (96) with more than 90% of pregnant women in the United States having measurable urinary levels of BPA (17). In addition to maternal urine and maternal plasma (20), BPA levels have also been detected in placentae (97), neonatal cord blood (20), amniotic fluid (98, 99) and fetal tissues (100), supportive of placental transfer (see Table 1). BPA is well known for its estrogenic activity (101); however, it has also been shown to have anti-androgenic activity and to modulate glucocorticoids, PPAR, and thyroid systems (102-104). Emerging data indicate that the replacement BPA analogues also have comparable endocrine-disrupting potential (93).

Phthalates are another group of EDCs used in the plastics industry for their ability to enhance the flexibility of plastics. The high-molecular-weight forms of diesters of phthalic acid are used as plasticizers in polyvinyl chloride products used as building materials, medical devices, and in food processing or packaging (96). Among these, phthalate di(2-ethylhexyl)-phthalate (DEHP) is the most widely used. Phthalates have been found in approximately 100% of humans tested (109), and according to the 2003 to 2004 NHANES study, 90% of pregnant women showed detectable levels of phthalates (17). Phthalates have been found in maternal urine (19, 110-112), maternal and cord blood (113, 114) and amniotic fluids (see Table 1). The endocrine-disrupting capability of phthalates includes antiandrogenic, estrogenic, antiestrogenic, and actions of glucocorticoid, thyroid, and metabolic receptors (115-117).

Industrial chemicals

In addition to the bisphenols and phthalates used in the manufacture of plastics, resins, and emulsifiers, several other chemicals are used as solvents, flame retardants, as well as in the manufacturing of consumer and industrial products including textiles, plastics, wire insulation, and automobiles. Among these, organohalogens, including polychlorinated biphenyls (PCBs) and brominated flame retardants (brominated flame retardants [eg, PBDEs], tetrabromobisphenol A [TBBPA], hexabromocyclododecane [HBCDD]) and perfluorinated alkylated substances (PFASs), are the most widely used (120). They are known for their persistence in the environment, bioaccumulation in living organisms, long-range transport beyond the geographical regions of their use, and long-term health effects in wildlife and humans (121, 122). Although many of these compounds have been banned or severely restricted for use in developed countries—some for as long as 4 decades—these PCB and PBDE congeners are still detectable in the environment and in populations (123). Precursors or metabolic intermediates of PFASs, which have received increased attention recently, include perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), and these have been detected in many food sources and drinking water (123). The 2003 to 2004 NHANES study found detectable levels of organohalogens from multiple classes in 99% to 100% of pregnant women (17). In addition, PFASs have been detected in maternal and neonatal cord blood (113, 122), placenta (122) and embryonic and fetal tissues (124, 125) (see Table 1). Similarly, PCB and PBDE have also been detected in maternal and cord blood, placenta and fetal tissues (126, 127) (see Table 1). The major effects of PCB and PBDE are associated with developmental neurotoxicity (128). Although the mode of disruption in endocrine systems is not known, interference with estrogen, androgen and thyroid signaling have been suggested (129). PFASs are known to cause developmental problems through disruption of endocrine and immune systems, and many PFASs are also known to have estrogen-like activities (123). The halogenated bisphenol in addition to acting through estrogen receptors has also been known to interact with PPAR (130).

Agrochemicals

Chemicals used in agriculture for crop protection, including pesticides, herbicides, fungicides and insecticides, comprise those belonging to organochlorines, organophosphorus, carbamates, pyrethroids, and neonicotinoids classes (131). Waterway transportation and harvesting of aquatic and marine food has also led to the introduction of another class of chemicals mainly used as anti-fouling agents, which include chemicals like tributyltin (TBT), chlorothalonil, dichlofluanid, diuron, and Zn pyrithione (132), that contaminate seafood. In addition, anabolic steroids, like the synthetic androgen, trenbolone, are used in the livestock industry to promote muscle growth and are routinely detected in various meats (133, 134).

Pesticides.

Organochlorine pesticides include biocidal chemicals such as DDT and its metabolites, hexachlorobenzene (HCB), lindane, and dieldrin (83). The use of DDT is now banned in many countries (135). The endocrine-disruptive functions for organochlorines include disruption of cytochrome enzyme expression through interference with AHR action (136) and alteration of thyroid hormone levels (137). Organophosphorus compounds, esters of phosphoric acids, such as chlorpyrifos and triazophos are among the most widely used insecticides. Organophosphorous compounds are inhibitors of the acetylcholinesterase enzyme involved in neuronal signal transmission (138). Metabolites are known to bind steroid and adrenergic hormone receptors and interfere with their signal transduction (139, 140). Other insecticides are chemicals within the carbamate ester functional group such as aldicarb, carbofuran, carbaryl, fenobucarb, oxamyl, and methomyl. Unlike organophosphorous compounds, these reversibly inactivate the acetylcholinesterase enzyme (141), and inhibit steroidogenesis as well as steroid hormone action (140).

Exposure to these chemicals can occur via household use of pesticide products, dietary exposure to pesticide residues, exposure to chemical drift from agricultural application, or agricultural use. Biomonitoring has found widespread exposures to agrochemicals in the human population. Despite being banned, DDT and its metabolites have been detected in maternal circulation (122) and placentae (142). In the CHAMACOS (Center for the Health Assessment of Mothers and Children of Salinas) cohort from the agricultural Salinas Valley in California, at least one of the pesticide metabolite of either carbamate, organochlorine, organophosphorus or pyrethroid was detected in 78% of the pregnant women (143) (Table 1).

Herbicides

Triazine herbicides that include atrazine, simazine, and propazine are used in large amounts to control weeds. Because of their widespread use in agriculture, it is of human exposure concern (144). Many earlier epidemiological studies focused on cancer risk; however, later on the developmental effects due to their endocrine-disruptive function began to be recognized (145). In the CHAMACOS cohort the herbicide triazine has been detected in at least 78% of pregnant women (143) (see Table 1). Endocrine-disrupting properties of atrazine have been linked to one or more of its metabolites and involves disruption of steroid synthesis through increased expression of aromatase (146).

Fungicides.

Dicarboximide, a fungicidal chemical like vinclozolin, is applied to various food crop, grass, and ornamental plants (143, 147). Studies relative to the presence of vinclozolin in maternal and fetal units are limited; one such study has shown the presence of vinclozolin in umbilical cord blood, indicating transplacental transfer (148). The effects of developmental vinclozolin exposures are well studied in animal models, in which it is demonstrated to have anti-androgenic effects (147, 149, 150), although data on its effects in human are limited (151).

Antifouling agents.

A well-known antifouling agent used in paints for the protection of ship hulls along with other derivatives (phenyltins and octyltins) is the potent biocide TBT, an organotin. Although the use of TBT has been banned since 2001 by the International Maritime Organization, its use continues unabated (152). Organotin compounds are also used as stabilizers in the manufacture of polyvinyl chloride plastics and polyurethane foams (152). They are commonly detected in human samples (153) and a few studies have also reported the presence of TBTs in human placentae (154, 155). Organotins are obesogenic endocrine disruptors as they are agonistic ligands for nuclear receptors retinoid X receptor (RXR) and PPAR γ (156). They have also been shown to affect placental steroidogenesis and chorionic gonadotropin secretion (157, 158).

Cosmetics and consumer products

Phthalates, parabens, and phenols, discussed earlier in the context of their use in plastics, are also used in a wide variety of consumer products such as perfumes, deodorants, soaps, shampoo, nail polish and cosmetics. The low-molecular-weight (LWW) phthalates such as dibutyl-phthalate (DBP) and dimethyl-phthalate (DMP) are commonly used, while parabens are used as preservatives in personal care products and cosmetics (164-166). The use of personal care products among pregnant women corresponded positively to the detection of phthalate metabolites in their urine, pointing to the risk of developmental exposure (167-169).

Triclosan and triclocarban are used as antimicrobials in products such as soaps, toothpastes, detergents, clothing, toys, carpets, plastics, and paints. As a result, these products have been detected in a wide variety of matrices worldwide. Triclosan and triclocarban persist in the environment and provide substrates for generating other toxic and carcinogenic compounds including dioxins, chloroform, and chlorinated anilines (170). They have been routinely detected in the urine of pregnant women (19). Among pregnant women from an urban population (Brooklyn, New York), the frequency of detection was 100% for triclosan and 87% for triclocarban (171). They were also detected in maternal blood, cord blood, and amniotic fluid, indicating transplacental transfer (172, 173) (see Table 1). The endocrine-disrupting role of triclosan is not well understood, but animal studies support a role in the disruption of adipocyte differentiation together with interference in estrogen, androgen, and thyroid hormone action (174, 175).

Pharmaceuticals and medical supplies

Exposure to endocrine disruptors can also occur during medical treatment because of the use of chemicals in the manufacture of medical equipment, such as tubing, syringes, pills, and implants. Some common medical exposures include phthalates, bisphenols, and parabens. In neonatal intensive care units, BPA was detected in almost 60%, and parabens in more than 85%, of medical devices tested (178). Phthalates are also present in equipment used in neonatal intensive care units, including platelet transfusion, and hemodialysis and extracorporeal membrane oxygenation devices (164).

Other pollutants

In addition to the EDCs discussed earlier, natural and synthetic chemicals such as PAHs, ozone, nitrogen oxides, and particulate matter (PM) are also released as a result of industrial activity. Such activities along with urbanization have led to environmental pollution of the ground, water, and air through industrial and domestic sludge and effluents. Biosolids, which are processed human sewage sludge, have been shown to contain various EDCs and use of this in agriculture and livestock is another source of exposure (185). Air pollution is also considered to contribute to endocrine disruptions with estrogenicity, antiandrogenicity, and thyroidicity demonstrated both in indoor and outdoor air (186). A major contributor of air pollution is PM, which may contain dust, soil, acids, organic molecules, and some metals. It is categorized according to the size of the particles as PM10 (2.5-10 µm), PM2.5 (<2.5 µm), and PM0.1 (<0.1 µm). The compounds present in PM are also known to have profound effects on the functioning of the endocrine system (187). Nanoparticles, synthetic materials that are gaining increased use in several industrial, consumer, and medical applications, have the potential to influence the endocrine system (188). Another source of exposure is the use of cigarettes. Tobacco smoke contains a mix of 7000 chemicals including EDCs such as benzene, a PAH, and vinyl chloride, a plasticizer (189). Smoking is associated with disruption of metabolic pathways via interaction with PPAR, thyroid hormone receptors, farnesoid X receptor, liver X receptor, and RXRs (190). Except for the detection of PAHs in maternal, placental, and fetal samples (see Table 1), the assessment of gestational exposure to other PM and airborne pollutants have been based on geospatial environmental assessment, occupation, and response to questionnaire (191-194).

Effect of Endocrine-Disrupting Chemicals on Pregnancy Outcomes

Because hormones play an important role in not only the establishment and maintenance of pregnancy, but also fetal development, EDCs that act as hormone mimics or interfere with their action have the potential to affect these variables. The association of EDC exposure with pregnancy outcomes is manifested as a range of adverse effects depending on the EDC, time of detection during pregnancy, and the compartment in which the EDC is measured. Such adverse outcomes range from preterm birth, fetal intrauterine growth restriction (IUGR), changes in birth weight and size, small for gestational age (SGA) babies, increase in gestational length, macrosomia, large for gestational age (LGA) babies, and congenital anomalies. Although many of these outcomes are established risk factors for adult-onset diseases (195-198), longitudinal studies linking the impact of EDCs in the maternal-fetal milieu to adult consequences will understandably not be available in the foreseeable future. Several pregnancy cohorts addressing the impact of maternal EDC exposures on gestational and birth outcomes are now following their children. The health consequences of gestational EDC exposure in children are briefly discussed in later.

In spite of the wealth of associative information available, determining the true impact of maternal/fetal exposure to EDCs on the range of pregnancy and infant outcome is challenging for several reasons. These include 1) differences in the timing of EDC measurement relative to pregnancy progression; 2) exposure level of EDC considering the non-monotonic responses to their exposure; 3) the matrix in which EDC measurement is undertaken, which include: maternal urine, maternal plasma/serum, placenta, cord blood and amniotic fluid; 4) background contamination due to ubiquitous presence of many EDCs; 5) differences in methodological approaches used for measuring EDCs; 6) single time point assessment during pregnancy and predominantly at term; 7) lack of longitudinal assessment of exposure to EDCs considering the organ-specific differences in susceptibility windows during gestation; 8) impact of fetal sex in modulating effects of EDCs 9) soundness of statistical approaches employed; and 10) lack of attention to confounders such as ethnicity, age, diet, prepregnancy weight, weight gain, and lifestyle factors. Furthermore, humans are exposed to multiple EDCs in parallel that may have additive, synergistic, or antagonistic effects and addressing one EDC at a time will not reflect true life exposure burden. The following sections address the impact of EDCs on various pregnancy outcomes. In addressing this, the association of gestational and birth outcomes based on geospatial environmental surveys, occupation, and/or response to questionnaire that lack direct measures of maternal or fetal presence of the EDCs are not considered. It needs to be recognized that the impacts of EDCs on pregnancy outcomes are based on observed associations between EDC presence in maternal/fetal compartments and outcomes, and hence should be viewed as providing biological plausibility and not establishing causality.

Gestational Length

Gestational length in humans, from the onset of the last menstrual period, averages 280 days. Although in 70% of all human pregnancies delivery occurs within 10 days of the estimated due date (199), deliveries can occur within 259 days; these deliveries are designated as preterm birth (PTB) (200) or can occur later than 280 days, which may lead to higher risk of macrosomia (201). In addition to higher risk for hospitalization, such outcomes are risk factors for adult-onset diseases (202, 203). Although the gestational age can be influenced by many factors including sociodemographic, nutritional, medical, obstetric, and environmental factors, emerging evidence points to environmental EDCs as contributory factors (204, 205). The association of EDCs with gestational age at delivery has been found to vary depending on the EDC, ranging from PTB (eg Pb, nickel [Ni], parabens, DDE, PFOS), lack of effect on gestational age (eg, triclosan, Bis(1,3-dichloro-2-propyl) phosphate, diphenyl phosphate), and longer gestational age (eg, Cr, lignans, BPA, PCB) (Table 2). In addition to effects from the parent compound, effects also vary based on the type of EDC metabolites they produce. For instance, among phthalate metabolites, maternal urinary DBP and diisobutyl phthalate (DiBP) were associated with an increase in PTB (206, 207) as opposed to mono(2-ethyl-5-hydroxyhexyl) phthalate (MEOHP) with a decrease in PTB (208). These varying outcomes, which depend on the type and amount of EDC, site of measure, and timing of assessment, substantiate the difficulties in making generalized statements regarding the impact of EDCs on gestational length and emphasizes the need to consider all these variables when addressing the effects of EDCs.

Table 2.

Endocrine-disrupting chemicals and their associations with pregnancy and birth outcomes

EDC	Gestational length		IUGR		Pregnancy complications		Birth weight		Congenital deformities
	Early	Mid to late term	Early	Mid to late term	Early	Mid to late term	Early	Mid to late term	Early	Mid to late term
Naturally occurring EDCs
Metals	Maternal urine: Se and Cu ↑ PTB (209)	Maternal urine: U (46); Ni (49); Pb (54, 200); Cd (53, 210) ↑ PTB Placental: Cr ↑ gestational length (212)	Maternal urine: As, Ba, and Pb ↓ fetal femur length (19)	Maternal urine: Cd ↑ IUGR (51) Pb ≠ IUGR (55) Maternal hair Mn ↑ chest circumference (238)	Maternal urine: Ni, As, Sb, Co, and V ↑ GDM (251)	Maternal term urine: Cr ↑ preeclampsia (45) Sb ↑ GDM (41)	Maternal blood and urine: As and Pb (257) Maternal urine Cd ↑ LBW (39)	Maternal urine: Cd ↑ LBW (48, 272) ≠ (57) Maternal blood: Cd ↑ LBW (211) Placental: Cd ↑ LBW and length (202) Pb ↑ LBW (43, 57); ↑ higher birth weight (259) ≠ birth weight (55) Hg ↑ LBW (52, 57) ≠ birth weight (274) As (267) and Hg (267) ↑ SGA		Maternal blood: Pb ↑ and Se ↓ (301) Pb ↑ (302) congenital heart defects Maternal blood As, Pb, Cd and Cu ↑ and Zn and Se ↓ neural tube defects (303)
Parabens	Maternal urine: ethyl paraben ↑ PTB (213)	Maternal TM3 urine and cord blood: BuPB ↑ PTB (214)			Maternal urine: methyl, ethyl, propyl paraben (254); butyl and propylparaben (253) ↑ GDM		Maternal urine: ≠ birth weight and length (273)	Maternal urine: ↑ LBW (262); ↑ birth length (275) Maternal TM3 urine and cord blood: BuPB ↓ birth weight (214) Maternal term urine: EtPB and BuPB ↑ birth weight; Et PB ↑ birth weight in ♂ (276, 278)	Maternal serum: propyl paraben ↑ cryptorchidism and reduced AGD (296)	Placental: propyl paraben ↑ cryptorchidism (304)
PAHs		Cord blood: (215) Placenta: (72, 216, 217) ↑ PTB				Maternal blood and fetal tissue: PAHS ↑ abortions (by 14 wks) (79)		Placenta: ↑ LBW (72) Maternal urine: 1-OHP ↑ LBW, birth length and head circumferences (277, 279) 2-hydroxy fluorene ↑ LBW (280)		Placenta: ↑ neural tube defects (81)
Phyto- estrogens		Maternal blood: genisten ↑ PTB (218) Maternal urine: lignans ↑ gestational length (219)						Maternal blood: ≠ birth weight (83)		Maternal urine isoflavones: ≠ fetal AGD (293)
EDCs in plastics
Bisphenols		Maternal urine: BPA ↑ gestational length (20) Maternal term urine: BPS ↑ gestational length in ♀ (220)	Maternal urine: BPA ≠ fetal measures (236)	TM2 maternal urine: BPA ↑ low birth length and head circumference (239) Term maternal urine: BPA ≠ fetal measures (236); ↑ birth length in ♂ (240) Maternal urine BPA: ↓ fetal weight and head circumference (241)	Maternal blood: BPA fetal loss (237) Maternal urine: BPA ↑ preeclampsia (249) ≠ glucose levels (252)	Maternal urine BPA ≠ preeclampsia (249) ↑ blood glucose (252) ↑ GDM (257, 259)	Maternal urine: BPA ↑ LBW (19, 20)	Maternal urine ↑ LBW (239, 243, 244) ≠ birth weight (236) Placental BPA ↓ birth weight (268) TM2 amniotic fluid BPA ↓ birth weight (99)	Maternal serum: BPA ↑ cryptorchidism and reduced AGD (296) Maternal urine: BPA ↑ implantation failure (305)	Placental: BPA ↑ cryptorchidism (304)
Phthalates	Maternal urine: DEHP ↑ PTB (206) ↑ gestational length (208, 221) MEHP and MEOHP ↓ PTB (208)	Maternal urine: DBP and DiBP Phthalate metabolites ↑ PTB (206, 207)	Maternal urine: MBzP and MnBP ↑ IUGR (in ♂); DEHP ≠ fetal measures (310)	Maternal urine: DEHP ↑ IUGR (242) TM2 phthalates ↑ decrease head and chest circumference (239)	Maternal urine: DEHP ↓ impaired glucose tolerance (246) MEP and MOCP ↑ blood glucose (250)	Maternal urine phthalate metabolites ≠ preeclampsia (249) MEP ↑ impaired glucose tolerance; DEHP ↓ glucose tolerance (256)	Maternal urine: ↑ birth weight (19)	Maternal urine: DEHP ↑ LBW (242) MBzP and MnBP ↑ reduced fetal growth parameters and ↑ birth weight (in ♂) (236)	Maternal urine: MEP ↑ implantation failure (305)	Maternal urine: DEHP ↑ reduced AGD (293, 295)
					Average TM1-TM3 MEP ↑ impaired glucose tolerance (255, 256) MEP and DEHP ↑ preeclampsia (249)	Phthalate (average TM1-TM3 urine) ↑ impaired glucose tolerance (255) Maternal TM2 and TM3 urine MBzP ↑ blood pressure (260)		MEHP and MEHHP ↓ in ♂ and MMP and MEP↓ in ♀ birth weight (281) Maternal TM3 urine: MMP, MBP, MEHP, MEOHP, and MEHHP ↓ birth weight (282) Cord blood and meconium: DBP ↓ birth weight, DEHP ↓ birth length (283)
Industrial chemicals
Organo-halogens	Maternal plasma PFAS (222) PFOS and PFNA ↑ PTB (223)	Maternal serum PCB ↑ gestational length (224, 225)		Cord blood: PBDE congeners ↑ FGR (235)		Maternal serum: PBDE ↑ GDM: (261)	PFAS ↓ birth weight (222, 284); PFAS and PFNA ↑ LBW (223)	Maternal serum: PCB (285); PCB, PFOA, HCB (286) ↑ SGA
Agrochemicals
Pesticides		Maternal adipose tissue DDT and maternal serum DDE ↑ PTB (226, 227)		Maternal urine: dialkyl phosphates ↑ reduced fetal weight and length (237)		Maternal plasma: DDT/DDE ↑ Hypertensive disorders: (248)	Maternal blood: DDT (287)	Cord blood: β-HCH ↓ birth weight and ponderal index (in ♂) (289)		Maternal serum: DDE ↑ hypospadias, cryptorchidism (297, 298) Placental: DDT and HCH ↑ neural tube defects (81)
		Maternal plasma DDE (218); cord HCH (228) ↑ PTB Maternal urine isopropyl-phenyl phenyl phosphate: ↑ PTB (in ♀) and ↓ PTB (in ♂); Bis(1,3-dichloro-2-propyl) phosphate and diphenyl phosphate ≠ gestational length (229) Maternal plasma DDT ↑ gestational length (230)					Maternal urine: parathion and diazinon (288) ↑ LBW	chlorpyrifos, diazinon, and propoxur ↓ birth weight and/or length (290) p,p′-DDE, total DDT, β-BHC; p,p′-DDT, p,p′-DDD, HCB and mirex ↓ birth weight (291) Maternal urine: isopropyl-phenyl phenyl phosphate ↑ LBW (229) Maternal plasma DDT ↑ birth weight and length (230)
Herbicides			TM2 maternal urine: atrazine ↑ reduced head circumference (161)
Fungicide								Cord blood: tributylin: ↑ LBW (148)
Antifouling agents
Personal care products
Anti-bacterial		Maternal TM3 urine and cord blood: triclocarban ↑ PTB; triclosan ≠ gestational length (214)			Maternal blood triclosan: ↓ (250) ≠ (176) GDM					Maternal and cord blood: triclosan ↑ Congenital malformations of circulatory system, eye, ear, face, neck, urinary system and musculoskeletal system (173)
Medical products
Pharmaceuticals	Diethylstilbestrol ↑ PTB (231, 232)						Diethylstilbestrol ↑ SGA (231)		Diethylstilbestrol: ↑ Cryptorchidism and hypoplasia of the penis (299)
Medical Supplies (Bisphenols and Phthalates)	see above sections for effects of bisphenols and phthalates

Abbreviations: ↑, increase; ↓, decrease; ≠, no association; ∑, sum; ♂, male; ♀, female; AGD, anogenital distance; CRP, C-reactive protein; DBP, dibutyl-phthalate; DDE, diphenyl dichloro ethylene; DEHP, phthalate di(2-ethylhexyl)-phthalate; DMP, dimethyl-phthalate; E₂, estradiol; E₃, estriol; Early, first trimester; EDC, endocrine-disrupting chemical; FSH, follicle-stimulating hormone; GDM, gestational diabetes mellitus; IFNG, interferon γ; IGF1, insulin-like growth factor; IL, interleukin; IUGR, intrauterine growth restriction; LBW, low birthweight; MDA, malondialdehyde; mid to late, second and third trimester; mRNA, messenger RNA; miRNA, microRNA; PAHs, polyaromatic hydrocarbons; PBDE, polybrominated diphenyl ether; PCB, polychlorinated biphenyl; PFAS, perfluorinated alkylated substance; PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonic acid; PTB, preterm birth; SGA, small for gestational age; T3, 3,5,3′-triiodothyronine; T4, thyroxine; TM, trimester; TNF, tumor necrosis factor; TSH, thyrotropin.

Intrauterine growth restriction

IUGR is defined as diminished in utero growth of the fetus documented by at least 2 intrauterine growth measurements (233). Normal fetal growth depends mainly on sufficient delivery of oxygen and nutrients via the placenta. Factors that can disrupt placental function, including EDCs (234), contribute to IUGR. As with gestational length, the impact of the EDCs on IUGR vary depending on the EDC studied, time of measurement, the maternal/fetal matrix in which it is measured, and fetal sex (see Table 2). For example, whereas placental Pb measure (55) was not associated with abnormal fetal measures, first-trimester maternal Pb was inversely associated with femur length (19). Maternal presence of Cd (51) and PBDE (235), phthalate metabolites MBzP and MnBP (226), and organophosphate pesticides (237) for the most part have been reported to be associated with reduced fetal weight (Table 2). In addition, a sex-specific association for first- and third-trimester urinary phthalate with IUGR became apparent only when stratified by fetal sex, with strong associations with risk for IUGR among male fetuses (236, 242). Although a standardized approach to assess IUGR across EDCs has not been employed, available evidence points to several EDCs negatively affecting estimated fetal weight and ultrasound assessed fetal measures (head, chest and abdominal circumference, femur and fetal length depending on the time of pregnancy, matrix of EDC measurement, and fetal sex) (see Table 2). Based on the DOHaD hypothesis, alterations in these fetal growth indicators have the potential to serve as surrogate markers for long-term health outcomes and hence are a valuable resource.

Maternal Complication of Pregnancy

The prevalence of medical problems in pregnancy is increasing (245) with early pregnancy disorders manifested as spontaneous abortions, and late pregnancy disorders as maternal hypertensive disease (preeclampsia [PE] and chronic hypertension) and gestational diabetes mellitus (GDM). While demographic and lifestyle factors contribute to these pregnancy complications, evidence is emerging that environmental chemicals may be additional contributors (246-248). For instance, women with higher levels of benzo[a]pyrene DNA adducts during the second trimester (77) and unconjugated maternal blood BPA at 4 to 5 weeks of gestation (249) have manifested higher odds of early pregnancy loss. On the other hand, for gestational hypertensive disorders, the impact varied from gestational hypertension with the presence of DDT/DDE (250) to PE with BPA, phthalates (251) and Cr (45) (see Table 2). Similarly, the association of EDC with GDM also varied from no associations (176) to an inverse association with the presence of triclosan (252) and strong associations with the detection of the metals Ni, As, Sb, Co, and V (253), BPA (254), parabens (255, 256) and phthalates DEHP and MBP (257). Furthermore, the association of BPA and DEHP with PE and GDM appeared to vary depending on when in pregnancy they were measured, with a strong association found between first-trimester levels, but not with mid and late-gestation levels (251) for PE. Similarly, in contrast to first-trimester BPA (254) and phthalate DEHP (258) showing no associations with GDM, second-trimester urinary BPA levels (254) and the average of first- and third-trimester levels of DEHP (257) were associated with increased blood glucose and GDM, respectively. Another study showed that the association of first trimester phthalate levels varied depending on the metabolite, with DEHP having no association, whereas MEP and MOCP were associated with increased maternal glucose levels (252). These studies emphasize the need for considering not only various EDCs but also their metabolites and timing of pregnancy in addressing risks.

Birth Weight and Size

Birth weight and size are important predictors not only of neonatal morbidity and mortality (252), but also of long-term health of the offspring, predicted by the DOHaD hypothesis (3). These variables can be influenced by several factors that include demographic, socioeconomic, nutritional and lifestyle factors (262), with recent evidence pointing to additional detrimental impact following gestational exposure to EDCs (43, 263-265). The impact of the EDCs on birth size and weight also varied depending on the EDC, time of pregnancy measurement, whether maternal or fetal measures were considered and fetal sex (see Table 2). For BPA, the association with birth weight varied from no association with first-trimester and third-trimester maternal urinary levels of BPA (236, 239, 266) or maternal term blood (267), to associations with low birth weight with increased levels of maternal first-trimester circulating (20), second-trimester amniotic fluid (99), and term placental (268) BPA levels (see Table 2). Some EDCs such as PFAS, PCBs, PBDEs, organochlorines, or organophosphates (43), and As (269) showed no impact or small effects on birth weight. For phthalates, as is the case with fetal weight, the association with birth weight appeared to depend on the metabolite studied: MEHHP and MEOHP metabolites were associated with decreases (242), whereas MBzP in males was associated with higher (236) birth weight. Similarly, metabolite and sex-dependent associations were also evident for PFOS, with one study finding higher PFOS in first-trimester maternal plasma associated with low birth weight in male infants (270), but another study finding associations for all maternal PFASs with low birth weight in female neonates (271). The association for Cd varied with the matrix analyzed, with maternal term urinary (48, 272) and placental (212) levels showing associations, whereas cord blood levels surprisingly showed no association with low birth weight (57). These studies continue to emphasize when and where during pregnancy EDCs are measured, the matrix they are measured in, the parent or metabolite assessed, and the sex of the newborn are important considerations in determining the impact of EDCS on birth weight.

Birth Anomalies

The evidence relative to the involvement of EDC in congenital defects is limited, with the majority of the evidence focusing on the susceptibility of male fetuses (292). Triclosan and triclocarban in maternal and cord blood at term were associated with congenital malformations of the circulatory system, eye, ear, face, neck, urinary system, and musculoskeletal system (173), although the sex specificity of these outcomes is not known. The maternal term (293) and cord blood (294) levels of DEHP were linked with reduced male anogenital distance, a marker for insufficient fetal androgenization, along with reduced penis size and incomplete testicular descent (295) (see Table 2). Similarly, maternal serum BPA and propyl parabens (296), and the organochlorine pesticides DDT and DDE (297, 298), were found to be associated with increased male cryptorchidism, hypoplasia of the penis, and reduced anogenital distance (see Table 2). These findings highlight the potential for EDCs to affect congenital male reproductive tract defects such as congenital hypospadias and cryptorchidism, and reduced anogenital distance, a known marker of masculinization (300). The impact of EDCs on female congenital anomalies are understudied and is an area for future investigation.

Offspring number and sex distribution

Sex ratio is measured as the ratio of male to female births and is considered to be about 1.06 (106 boys for every 100 girls born) (306). Although sex determination is genetic, sex ratio of birth outcomes appears to be influenced by numerous factors and may be dictated by seasonal changes, maternal age, interpregnancy intervals, family size, birth order, length of follicular phase and moment of conception, age of the fathers, in vitro fertilization, and maternal diabetes (307). Growing evidence, particularly in fish, amphibians, and reptiles, implicates anthropogenic disturbance, such as environmental chemical contamination, in overriding the genetic determination of gonads and altering the sex ratio (308). However, to what extent these observations can be expanded into higher species including mammals remains to be determined. Limited evidence available from analysis of birth records spanning many decades has pointed to significant decreases in the proportion of boys born in industrial countries, including the United States, Denmark, Finland, and the Netherlands (306, 309-311). In addition, some studies have linked changes in sex ratio to geographical and occupational exposure, for example, the use of pesticides in Russia (312). Although these observations raise the possibility for environmental chemicals to have the potential to affect sex ratios, direct evidence in humans is still lacking and would require decades of work involving larger cohorts.

Dose Effect of Endocrine-Disrupting Chemicals on Gestational and Birth Outcomes

Dose-response relationships are central to toxicological assessments of various hazards and risk assessment of environmental toxicants are typically based on linear dose-response effects. Some reports of associations with gestational and birth outcomes with EDCs appear to have linear dose-response relationships. For instance, early pregnancy loss was associated with presence of DDT in maternal blood in a dose-dependent manner, with relative odds increasing by 1.17 with every 10-ng/g increase in serum total DDT (313). In contrast, midpregnancy maternal urinary BPA levels were associated with decreasing anogenital distance, with higher quartiles showing strong associations (314). Likewise, dose-specific effects diminishing birth weight were evident with maternal first-trimester blood levels of BPA (20) and PFAS (271), urinary levels of BPS (19) and heavy metal V (315), maternal third-trimester urinary levels of As (37), methyl paraben (316), and 4-tert-octylphenol (317). In contrast, dose-dependent increases in maternal first-trimester urinary levels of the phthalate metabolite MCPP were associated with an increase in birth weight (19). Linear dose assessments, however, have usually been based on establishing quartiles without consideration of the entire dose range.

Hormonal responses in general are nonlinear and manifest nonmonotonic dose-response with U- or inverted U-shaped, or “biphasic,” dose-dependent effects and this kind of relationship is also evident with EDCs (318) both in animal models and epidemiological assessments in humans (319). For instance, an inverse U-shaped association was evident between maternal urinary BPA and birth weight, whereas the opposite, a U-shaped association, was present between phthalate MCPP and MECPP, and birth weight (275). Non-linear, but dose-dependent effects of PTB suggestive of a nonmonotonic relationship were also evident, with maternal circulating levels of PFAS (222, 223) and early pregnancy urinary levels of V (315). Such nonmonotonic dose effects are not confined just to birth outcomes. For example, reported inverse U-shaped relationships between maternal serum concentrations of PCBs with vitamin D3 (320), cubic spline association of maternal first-trimester circulating As levels with impaired glucose tolerance (321), and a U-shaped dose–response relationship between maternal third-trimester urinary MnBP and placental steroidogenic gene expression (322), are indicative of nonmonotonic relationships of EDC with maternal and fetal outcomes. These findings emphasize the dose-specific nature of EDC impact and caution against assuming linearity in dose response as often done in toxicological studies.

Effects of Endocrine-Disrupting Chemical Mixtures on Gestational and Birth Outcomes

Most of the studies discussed earlier have assessed the associations of gestational and birth outcomes with the maternal or fetal presence of an individual EDC. Considering that humans, in real-life scenarios, are routinely exposed to multiple chemicals of different classes with similar and differing signaling mechanisms, the cumulative effects of EDC exposure are of ultimate importance. Working via common or diverse receptor pathways, these environmental chemicals may lead to additive, synergistic, or antagonistic outcomes (see Fig. 1), with the overall effect dictated by the net effect of different chemicals in the mixture (323, 324). For example, combinations of 11 xenoestrogens were effective in producing an effect such as increased activity of yeast estrogen receptor α reporter, while each EDC tested at the same cumulative dose level as in the overall mixture failed to produce any effects (325). This emphasizes the clear need to assess the effects of EDC mixtures, although the current regulatory guidelines governing allowable concentrations of environmental EDCs have been developed on a chemical by chemical basis (326). Because of this, mixture risk assessment (MRA), defined as the determination of the cumulative risk to human health or the environment due to exposure to different chemicals via multiple routes, is gaining prominence (326). Initial studies of MRA of maternal exposures involved the same classes of chemicals (eg, phthalates) or chemicals with the same mode of action (eg, xenoestrogens). One study found that whereas analysis of individual phthalate metabolites showed a modest association with low birth weight for each metabolite, inclusion of all metabolites in the analysis increased the strength of the association (327), indicating the combined synergistic effects of phthalate metabolites. Such associations were also evident for other EDC classes in which early gestational presence of persistent organic pollutants in maternal plasma showed weak associations with birth anthropometry when assessed individually, but when analyzed as a class of EDCs such as organochlorine pesticide, PBDE, or PCB mixtures, manifested strong associations with different birth measurements (328, 329). Similarly, whereas individual PAH metabolites in maternal second-trimester urine showed no individual association with birth weight and birth measures, the sum of the PAH metabolites showed a significant interaction with cephalization index (head circumference to body weight ratio), a measure of brain growth (330). Measures of individual organochlorines showed both positive and negative associations depending on the fetal measure, whereas as a mixture they mainly manifested as negative associations (329). These reports are consistent with chemicals with similar action or classes having additive or synergistic effects on pregnancy and birth outcomes.

Assessing a single class of EDC for cumulative effects, however, is not effective in determining the MRA, because humans are exposed to different EDCs belonging to categories of metals, plastics, plasticizers, persistent organic pollutants, and agrochemicals, among others. About 40 to 50 different EDCs have been detected in maternal and fetal samples (17, 19), although these numbers reflect only a fraction of the exposome. As these chemicals are of divergent characteristics with similar as well as dissimilar modes of action (see Fig. 1), statistical modeling through cluster- and principal–component based analyses are being employed to gain an understanding of the impact of complex chemical mixtures (18, 331). The approach of statistical modeling in MRA is in its infancy and standardization of this approach is needed as various factors such as biomonitoring approaches, interaction between chemicals, geographical locations, socioeconomic status of the participants all influence the outcome (332). A very limited number of studies have assessed the maternal presence of EDCs as mixtures and their effects on gestational and birth outcomes. For instance, modeling of 43 EDCs present in the maternal milieu through cluster and principal component analysis (331) revealed an inverse association of the exposome with birth measures (277). Another study found clusters of chemicals containing phenols, phthalate metabolites, several metals, organophosphate and organochlorine pesticides, PCBs, and several PFAS were associated with low birth length (277). Likewise, a principal component containing oxychlordane, trans-nonachlor, benzophenone-3, triclosan, As, dialkylphosphate (DAP), and PCBs was associated with reduced birth length only (277). Similarly, whereas maternal first-trimester urinary EDCs individually showed an association with low levels of maternal and cord blood oxidative stress markers, these EDCs modeled as mixtures through principal component analysis showed both positive and negative associations in a sex-specific manner (333). Analysis of associations of EDC mixture with inflammatory cytokines also showed both positive and negative relationships that varied depending on the EDC mixture and cytokine (334). Emerging studies in humans exposed to complex chemical mixtures used in fracking industries or air pollution through geographical proximity or occupation, have also manifested poor gestational and birth outcomes (335-340). Animal studies substantiate causal relationships by demonstrating the association of exposure to fracking chemical mixture (341) or biosolids (185) with adverse developmental health and reproductive outcomes. These emerging data point to the differing impact of the maternal-fetal exposome as a whole on gestational and birth outcomes compared with individual EDC components.

Mechanisms Through Which Endocrine-Disrupting Chemicals Affect Pregnancy and Birth Outcomes

Maternal and fetal milieus are tightly controlled throughout pregnancy to maintain the energy demands of the growing fetus. The placenta serves as a conduit between the mother and fetus to maintain fetal homeostasis. EDCs affect pregnancy not only by acting directly as hormonal agonists/antagonists to influence endocrine functions, but also indirectly by disrupting maternal, placental, and fetal homeostasis.

Altered Maternal Milieu

The effects of EDCs on the maternal environment may be mediated by having an impact on the inflammatory and oxidative state, hormonal milieu, metabolomic profile, and microbiome.

Inflammation

The inflammatory state varies during the entire course of pregnancy, with 3 separate immunological states described: a proinflammatory state during early pregnancy with high levels of cytokines, an anti-inflammatory phase during midgestation and a return to a proinflammatory state during the late gestation and term (342). Labor/delivery occurring spontaneously is also characterized by a strong inflammatory state with an influx of immune cells along with increased levels of cytokines (342, 343). Disruption of this delicate balance can lead to pregnancy complications such as implantation failure, recurrent pregnancy loss, intrauterine growth restriction, PTB, PE, and GDM (344-347).

EDCs are known to modulate the immune system through their action on the immune regulatory network, cellular and humoral response, survival, maturation, and cytokine production by immune cells (348, 349). In line with this, several reports, including ours (334), have shown that EDCs such as BPA (350) and triclosan (351) increase proinflammatory cytokines, including interleukin 6 (IL-6) and PBDE and PFAS increase IL-6 and tumor necrosis factor α (TNF) in the maternal circulation (352). In addition, markers reflective of the inflammatory state are also associated with maternal exposure to EDCs. These include increases in 1) maternal circulating C-reactive protein (CRP), a marker of systemic inflammation with increased urinary BPA analogues BPB and BP3 across pregnancy (351) and 2) 8-iso-prostaglandin F2α, a product of enzymatic sources linked to inflammation, with multiple LMW and high-molecular-weight (HMW) phthalate metabolites in maternal third-trimester urine (353). Similarly, principal component analysis modeling of maternal first-trimester urinary EDC mixtures showed both positive and negative associations with inflammatory cytokines depending on the chemical mixture and the cytokine measured (334). For example, principal component grouping with higher weighting for metals and phthalates in maternal urine was associated with higher levels of IL8 and interferon γ (IFNG) in maternal first-trimester plasma. In contrast, principal component grouping with lower weighting for metals was associated with lower levels of IL1B levels in maternal term plasma (334). This direct and indirect evidence on the EDC influence on inflammatory state during pregnancy mostly comes from assessments carried out during late pregnancy or term for the majority of EDCs, with very limited information available from early pregnancy (Table 3).

Table 3.

Early and late gestational presence of endocrine-disrupting chemicals and their associations with maternal and fetal mediators

EDC classes	Inflammation		Oxidative stress		Hormonal changes		Metabolomic changes		Microbiomic changes
	Early	Mid to late term	Early	Mid to late term	Early	Mid to late term	Early	Mid to late term	Early	Mid to late term
Metals	Maternal: Se, Mo, Cd, Ni, Cu, Zn, Pb ↑ MCP3 and IL8 (334)	Maternal: Se, Mo, Ni ↑ maternal MCP3 (334) Maternal W and U ↓ cord MCP1 (334) Maternal Mo, Ni, Cd, Zn ↑ cord MCP1a and MCP3 (334) Maternal As ↑ IL1B, IL8, IFNG, TNF (355)		Maternal Pb and Cu ↓ cord nitrotyrosine and chlor- tyrosine (333)	Maternal V, As, Pb ↓ free T3 (36) Maternal As, Se, Mn, Ni, Sb ↓ free and total T3 or T4 (381) Maternal Ce and Yb ↓ cord TSH (382)		Maternal Cd ↑ urine L-cystine, L-tyrosine, and dityrosine; ↓histamine, and uric acid (256) ↑ uric acid (399)	Maternal toenail As level ↑ butyry- lqlycine and tartrate (400) Maternal TM2: higher Cu and lower Mo: ↑ glucose level (394) Maternal Ar ↑ TM2 cord 17- methylstearate, laurate (12:0) and 4- vinylphenol sulfate (400)	Maternal Methyl Hg Gut microbiome composition changed (416)	Maternal Methyl Hg No effect on gut microbiome composition (416)
Parabens		Maternal urine: butyl paraben ↓ CRP (351) Ethyl paraben ↓ IL1B (356)		Maternal urine: butyl, ethyl, methyl, propyl paraben ↑ 8-hydroxy- deoxyguanosine and 8- isoprostane (362) Maternal term urine: Methyl and ethyl paraben ↑ malondealdehyde (MDA) and 8- hydroxydeoxy- guanosine (63)		Maternal butyl paraben ↓ E2 and free T4 (375); ↑ total T4 (376) Maternal methyl and propyl paraben ↓ mid-pregnancy E2 and ↑ late-pregnancy E2 (375) Maternal propyl paraben Maternal: ↓ free T4 (376) Maternal term urine: EtPB ↑ cord blood triiodothy- ronine levels (278)	Maternal urine: Parabens affect purine metabolism, fatty acid β-oxidation, and other pathways: methyl paraben ↑ hypoxanthine and 7- methylxanthine; ethyl paraben ↑ benzoic acid; propyl paraben ↑ trimethylamine N-oxide, dihydrobiopterin, and 2,6-dimethy- lheptanoyl carnitine (401)	Maternal butyl paraben ↑ TM2 glucose Propyl paraben ↓ glucose (353)
PAHs			Placental PAH DNA adducts: ↑ TBARS and 7-ethoxy- coumarin O-deethylase, ↓ glutathione S-transferase activity (367)	Maternal urine: 1- hydroxypyrene ↑ MDA (366, 367); placental 1- hydroxypyrene: ↑8-OHdG (75)		Maternal Hydroxylated PAH ↓ E2 and T (280)
Phytoestrogens						Maternal equal ↑ E3 (84) Maternal matairesinol, enterodiol and entero- lactone ↑ urine hydroxylation products of estrogens (219)
Bisphenols		Maternal BPA ↑ IL6 (350) Maternal BP3 ↓ CRP (351)	Maternal BPA ↑ 3-nitrotyrosine (364)	Maternal BPA ↑ cord 3-nitrotyrosine (364) Maternal BP3 ↑ 8-hydroxy- deoxyguanosine (8-OHdG) and 8-isoprostane (363) Maternal BPS ↑ 8-isoprostane (363)	BPA: baseline cortisol↑ in female, baseline cortisol↓ in male (383) Maternal BPF: ↑ free T3 (384)	Maternal BPA ↓ maternal TSH (369, 370); ↑ maternal free T4 (370) ↓ maternal T (374) ↑ cord E2 (294) Maternal BPA ↓ CRH (364) Maternal BPF ↑ free T4 (364)	BPA ↑ maternal palmitic acid; oleic acid and total free fatty acids (364)	Maternal BPA ≠ glucose levels (249) ↑ urine - endocanna binoid, palmitoleamide and lysophosphatidy- lethanolamine 18:0 (402) Cord ↓leptin and ↑high-molecular-weight adiponectin (445)
Phthalates	Maternal MnBP, MBzP, MCOMHP, meCPP, MEHP ↑ MCP3 and IL8 (353)	Maternal MnBP, mCINP, meCPP, MEHP, MEHHP, MEOHP ↑ MCP3 (353) Maternal MnBP/ MEHP ↑ cord MCP1a /MCP3 (353) Maternal MnBP, MIP, MNP ↑ 8-iso-prostaglandin F2α (351)	Maternal MCPP and MCINP ↓ chlor-tyrosine (333)	Maternal MBzP and MCOMHP ↓ cord nitro-tyrosine (333)	Maternal DEHP slower rise in hCG (305) Maternal MiBP, MEHP, MEOHP ↑ E1 and E2 (378) Maternal MBzP ↑ E2 (378) Maternal MCNP and DEHP ↓ free T (378) Maternal urine: MnBP, MBzP, MCIP ↑ hCG (386)	Maternal MEHP and DEHP ≠ cord E2, T, FSH, LH, T3, T4, and TSH (282) Maternal MEP: ↓ maternal total T4 (372); ↑ total and free T (377) Maternal DEHP: ↓ maternal total and free T (377)		Maternal MEP ↑ blood glucose (396) Maternal DEHP ↓ triglycerides, palmitic, oleic, linoleic, and α-linolenic acids (397) Maternal MEP, MnBP, MiBP, DEHP ↑ nicotinamide mononucleotide, cysteine T2, cystine, and L-aspartic acid (398) MiBP, MnBP, and DEHP ↓ Cord leptin (445)
						↓ cord cortisol, cortisone and glucocorticoid/adrenal androgen ratio, and ↑ cord DHEA and DHEA/androstenedione ratio (439) Maternal MBszP ↓ TSH (372) Maternal MEP, MnBP, MiBP, DEHP ↓ total T4 Cord: ↓ TSH and total T4 (372) ↓ androstenedione, T and DHEAS and ↑E2 and E3 (114)
Organohalogens		Maternal PBDE and PFAS ↑ IL6 and TNF (352)			Maternal serum HCB and PCB congeners ↓ total T3 and ↑ free T4 (387) PFOS: ↓ maternal TM3 cortisone (388)	PFOS and PFNA ↓ newborn TSH (430) PFOA and PFOS Maternal hypothyroid (389) Cord: PFOS ↑ DHEA and cortisol; PFOA ↓ DHEA (441) PCB ↓ newborn T3 and T4 (436,437) ↓ T and ↑ E2 (443) PCDF ↑ maternal 4-hydroxyl E2; PCDD ↓4-hydroxyl E2/ 2-hydroxyl E2 ratio (390) PBDE ↑ cord TSH (436) ≠ maternal and cord E2 (444)		PBDE ↑ maternal fasting glucose (395) PFAS ↓ maternal glucose (403) PFOS ↑ maternal glucose; PFOS metabolite inverse-U shaped association (404)
Agrochemicals		Maternal HCH, DDD, DDE ↑ IL6 and IL4 (357) Maternal plasma PBDEs ↑ IL6 and TNF; PFAS ↑ IL6; hydroxylated PBDE metabolites ↓ IL10 (352); ≠ CRP (354)				DDE and HCB maternal (366) and cord (429) ↓ total T3 and T4; ↓ TSH (389) DAP ≠ maternal and cord TSH, T3 and T4 (374) Maternal pyrethroids ↑ TSH (373) Chlordanes, cis-HCB, heptachlor epoxide, Mirex, and toxaphenes Cord: ↓ T, cortisol, cortisone, prolactin, ↑ DHEA, FSH (♂) (442) DDT Cord: ↓ DHEA and ↑ cortisol (♀) (442)		Maternal urine pesticides: ↑ glycine, threonine, lactate and glycerop- hosphoc- holine and ↓ citrate (405)
Antifouling agents
Personal care products		Maternal urine: Triclosan ↑ CRP, IL10, TNF; 2,5- dichlorophenol ↑ CRP (356)		Maternal triclosan ↑ 8-hydroxy- deoxyguanosine (363)	Maternal triclosan ≠ TSH, T3 or T4 (385)	Maternal triclosan ↑ E3 (376) Maternal triclocarban ↑ T3 and T3/T4 (375)
Pharmaceuticals	see sections above for effects of bisphenols and phthalates

Abbreviations: ↑, increase; ↓, decrease; ≠, no association; ∑, sum; ♂, male; ♀, female; CRP, C-reactive protein; DDE, diphenyl dichloro ethylene; DEHP, phthalate di(2-ethylhexyl)-phthalate; E₂, estradiol; E₃, estriol; Early, first trimester; EDC, endocrine-disrupting chemical; FSH, follicle-stimulating hormone; IFNG, interferon γ; IGF1, insulin-like growth factor; IL, interleukin; MDA, malondialdehyde; mid to late, second and third trimester; mRNA, messenger RNA; miRNA, microRNA; PAHs, polyaromatic hydrocarbons; PBDE, polybrominated diphenyl ether; PCB, polychlorinated biphenyl; PFAS, perfluorinated alkylated substance; PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonic acid; T3, 3,5,3′-triiodothyronine; T4, thyroxine; TM, trimester; TNF, tumor necrosis factor; TSH, thyrotropin.

Oxidative stress

Metabolic activity leads to the production of reactive species (reactive oxygen/nitrogen species) that are neutralized by a cellular antioxidant defense mechanism, failure of which leads to oxidative stress. During pregnancy, physiological concentrations of reactive species participate in placental development (358). Increases in metabolic activity of the placenta and the growing fetus (359) increase production of reactive species (360) that are offset by corresponding increases in antioxidants (361). Failure of this protective mechanism is associated with adverse pregnancy and birth outcomes such as miscarriage, PE, fetal growth restriction and preterm labor (362).

The presence of EDCs in maternal and fetal milieu were associated with elevations in biomarkers of oxidative stress. For example, increases in oxidative stress biomarkers hydroxydeoxyguanosine (8-OHdG), a marker of DNA damage and 8-isoprostane, a biomarker for lipid peroxidation with maternal urinary dichlorophenols, benzophenone-3, triclosan, and parabens across pregnancy, 8-8-isoprostane with BPS (363), and nitrotyrosine- a marker of cell damage and inflammation with maternal urinary BPA (364) have been reported (see Table 3). In contrast, the presence of HMW phthalate metabolites at 13 weeks of gestation were associated with low levels of 8-isoprostane (365), maternal first-trimester urinary phthalate metabolites MCPP and MCINP with low levels of maternal first trimester chlortyrosine, a marker of myeloperoxidase-catalyzed oxidation, and MBzP and MCOMHP with nitrotyrosine (333). As opposed to these individual associations, analysis of maternal first-trimester urinary EDCs as mixtures has revealed associations varying with sex, including both positive and negative associations depending on the chemical composition of the EDC mixture and biomarker assessed (333). For example, principal component analysis with lower weighting for maternal first-trimester urinary metals and phthalates, was associated with higher levels of chlortyrosine in maternal first-trimester plasma. These studies point to the impact of EDCs on the pregnancy oxidative state. However, as with inflammation, outside of EDCs such as bisphenols and phthalates, information is limited on the effect of several EDC classes on oxidative stress in pregnancy.

Hormones and growth factors

Pregnancy is associated with profound physiological changes that are driven by dynamic changes in maternal, placental, and fetal hormones. Hormones control pregnancy at every stage, from the process of implantation (progesterone) and maintenance of pregnancy (chorionic gonadotropins and progesterone), to parturition (glucocorticoids and corticotropin-releasing hormone, as well as estradiol [E₂] and oxytocin) (368). The physiological hormonal homeostasis during pregnancy is tightly regulated (369), and available evidence so far indicates that EDCs adversely affect pregnancy hormonal homeostasis (see Table 3). For instance, EDCs have been shown to disrupt secretion of anterior pituitary thyrotropin (TSH), which regulates the release of 3,5,3′-triiodothyronine (T3) and thyroxine (T4) secretion, which are important for fetal development, particularly the central nervous system (see Table 3). Studies have also indicated that maternal levels of TSH and thyroid hormones were influenced by third-trimester maternal EDCs in an EDC-specific manner with BPA (370, 371), MBzP (372), and organochlorine pesticides (366) decreasing, pyrethroids and triclocarban increasing (373), and organophosphate DAP metabolites (374) having no association with TSH levels. Similarly, whereas BPA (370) and BPF (375) were shown to increase free T4, phthalate metabolites (MEP, MnBP, MiBP, and DEHP metabolites) decreased maternal total T4 (372).

Disruptions in maternal steroidal hormones particularly sex steroids—key regulators of sexual differentiation and sexual dimorphism—were also found to be associated with EDC exposure (see Table 3). For instance, late pregnancy butyl paraben has been found to be associated with a decrease in maternal blood levels of E2 (376) as opposed to BPA being associated with decreased levels of testosterone (T) in maternal blood (375). Associations for some EDCs depended on the time of measurement during pregnancy or the EDC metabolite with methyl and propyl paraben linked with decreased and increased maternal serum E2 levels during mid- and late pregnancy, respectively (376). Similarly, urinary MEP levels were associated with increased levels of total and free T (377), in contrast to MCNP and DEHP levels being associated with decreased levels of free T (378).

Gestational EDC exposure was also associated with disruptions in maternal growth factors, particularly angiogenic factors, which have a bearing on placentation and placental function because the placenta is an organ with extensive vascular network promoting nutrient and waste exchange between the mother and fetus. The disruptive effects of EDCs that were manifested at the level of members of placental growth factors (PIGFs) and vascular endothelial growth factors (VEGFs) included (i) maternal urinary DEHP or BPA throughout pregnancy with decreased maternal circulating PlGF and soluble Flt1 (VEGF receptor) to PlGF ratio (379, 380); (ii) maternal third-trimester urinary PAH metabolites 2-naphthol, 2- and 3- hydroxyphenanthrene (PHE), and 4-PHE with decreased PIGF; (iii) third-trimester maternal circulating 9-PHE with increased maternal circulatory soluble Flt1 in (390); and (iv) maternal third trimester urinary metals both individually and as mixtures with changes in maternal angiogenic factors (45) (see Table 3). These findings suggest that EDCs have the potential to disrupt the maternal hormonal balance critical for proper maintenance of maternal, fetal, and placental function.

Metabolome

Considering that hormones help maintain general homeostasis, the impact of EDCs on the metabolic milieu may be mediated indirectly via disruption of the hormonal milieu. Alternatively, EDCs can directly affect the metabolic milieu by altering the enzymes involved in xenobiotic metabolism (391). The negative impact of EDCs on GDM and hypertensive disorders discussed earlier (Table 2) have resulted from disruption in glucose homeostasis (392) and renal function (393), respectively. That multiple EDCs affect glucose homeostasis (see Table 3) were supported by findings that Cu (394), butyl paraben, PBDE (395), and the phthalate metabolite MEP (396) are associated with higher second-trimester maternal blood glucose levels, key contributors to GDM. Likewise, EDCs also affect other aspects of metabolism such as lipid and amino acid metabolism. For example, first-trimester BPA was associated with higher levels of circulating maternal palmitic acid, oleic acid and total free fatty acids (364). In contrast, whereas second- and third-trimester urinary DEHP phthalate metabolites were associated with decreased levels of triglycerides, palmitic, oleic, linoleic, and α-linolenic acids (397), phthalate metabolites MEP, MnBP, MiBP, and DEHP were associated with higher levels of nicotinamide mononucleotide, cysteine, cystine, and L-aspartic acid (398). Therefore, the effects of EDCs in disrupting the balance of metabolites required for fetal growth, differentiation, and maintenance of metabolic and oxidative homeostasis may in part contribute to their negative effects on pregnancy outcomes.

Microbiome

Microbial organisms inhabit the skin, and gastrointestinal and urogenital tract (406) and therefore are part of the maternal compartment. With the identification of microbiome colonization in the placenta and fetal gut (407, 408), it is now clear that the fetal compartment also harbors a microbiome. The presence of microbiota aids in driving many host responses and thereby can influence the host phenotype in health and disease states (409). Growing evidence has also indicated that the microbiome can influence maternal, birth, and long-term child health outcomes (410, 411). The composition of microbiota has been shown to be influenced by host health and diet, with emerging evidence indicating EDCs also play a role (412, 413). The interaction between EDCs and host microbiota has been shown to be complex: While EDC exposure disrupts the microbiome or causes dysbiosis (eg due to antimicrobial actions of parabens or triclosan [403, 404]), the microbiome-produced enzymes and metabolites affect EDC biotransformation and its bioavailability (414).

Evidence that microbiota may be protective against EDCs comes from a study that showed consumption of probiotic yogurt by pregnant women reduces the increased mid-to-late pregnancy Pb and As (415). While this study suggests that the microbiota has the potential to modulate EDC bioavailability, limited data are also available suggesting that the presence of maternal EDCs induces changes in maternal microbiota. One study reported maternal blood levels of methyl Hg to be associated with changes in gut microbiota during the first trimester, but not the third trimester (416) (see Table 3). Most information relative to the impact of EDCs on microbiota come from animal models. For instance, perinatal treatment of mice with Pb (417) and BPA (418), as well as exposure to PM2.5 (419), resulted in gut dysbiosis both in dams and offspring. Considering that the mode of delivery between vaginal and cesarean delivery has a demonstrable effect on the infant microbiome (420, 421) and infant health outcomes (422, 423), the finding that maternal EDCs have the potential to alter the vaginal microbiome (424) suggests they can also affect the infant microbial transfer. This field is in its infancy, but limited available data from human and animal studies point to EDC exposure affecting maternal and fetal microbiota and thereby influencing pregnancy, birth, and child health outcomes in humans.

The Effect of Endocrine-Disrupting Chemicals on Fetal/Neonatal Milieu

As discussed earlier, environmental EDCs can affect not only the maternal milieu but can also cross the placenta to reach the fetus. Fetal compromise may therefore be mediated either through direct action of the EDCs on the fetus or indirectly by the effects of EDCs on the maternal milieu and/or placental function. In humans, the majority of assessment of changes in the fetal/neonatal milieu were accomplished by detecting changes in umbilical cord blood collected at birth. As such, the effects of EDCs on the fetal developmental trajectory and changes in the fetal hormonal milieu throughout gestation are not available.

Inflammatory and oxidative states

Inflammation and oxidative stress states in the fetus are strongly associated with PTB and low birth weight (425, 426). Considering that inflammation and oxidative stress go hand in hand with the developing embryo and fetus having low antioxidant capacity, excessive production of reactive oxygen species can lead to an adverse oxidative and inflammatory state that is detrimental to fetal growth (427). In spite of its importance, studies addressing the impact of EDCs on cord blood levels of inflammatory and oxidative stress markers have been limited to outcomes relative to maternal levels of EDCs (see Table 3). Available findings indicate: 1) a decreased inflammatory cytokine CCL2 association with W and U; 2) decreased IL33 and thymic stromal lymphopoietin (TSLP) with presence of Pb, PCB118, DDE, DEP, and DETP (428); 3) increased CCL2 and CCL7 with Ni, Cd, Zn, MnBP and MEHP (327); and 4) increased TNF with PBDE (429) in maternal urine or circulation (see Table 3). Airborne PM10 in term maternal blood was linked with contrasting changes in proinflammatory and anti-inflammatory cytokines with increased IL1B and decreased IL10 in cord blood (430). One study that addressed cord blood levels of EDC with outcomes found the pesticide chlordane (CHD) in cord blood was associated with lower levels of the proinflammatory IL-1B and permethrin with low levels of anti-inflammatory cytokine IL-10 (431).

Available evidence has also pointed to maternal EDC exposure–specific changes in oxidative stress biomarkers in the neonatal milieu (432). These include low cord blood levels of oxidative stress biomarkers nitrotyrosine and chlor-tyrosine with the presence of metals Pb and Cu, and nitro-tyrosine with the presence of phthalate metabolites MBzP and MCOMHP in maternal first-trimester urine (333). In contrast, maternal first-trimester plasma levels of BPA were associated with high levels of nitrotyrosine in cord blood (364). On the other hand, PAH and PM2.5 in maternal blood during late pregnancy showed no association with markers of oxidative DNA damage (8-oxo-7,8-dihydro-2′-deoxyguanosine) and lipid peroxidation (15-F2t-isoprostane) in newborn blood (433). These findings point to maternal EDCs having the potential to disrupt cord blood inflammatory and oxidative stress status.

Hormones

In addition to genetic factors, autocrine, paracrine, and endocrine networks of hormones and growth factors shape the development of the fetus by coordinating maternal-placental-fetal interactions and fetal maturation (434). Fetal endocrine function begins early in gestation and continues to term and is able to adapt to maternal changes such as hypoxemia and hypoglycemia (435). As such, exposure to EDCs that could disrupt the fetal endocrine milieu could also contribute to altered fetal growth with adverse gestational, birth, and long-term health outcomes. Considerable evidence exists relative to the impact of maternal EDCs on reducing newborn thyroid status (see Table 3). These include 1) decreased cord blood levels of TSH with presence of Ce and Yb (382); phthalate metabolites MEP, MnBP, MiBP, DEHP (372) and DDT (436), 2) decreased T3 and T4 with presence of PCB (437) and DDT (436); 3) decreased free and total T3 with presence of HCH; 4) reduced free and total T4 but increased TSH with presence of CHD in cord blood (436) and reduced total T4 with MEP, MnBP, MiBP, and DEHP metabolites (372) (Table 3). In contrast, no association of cord blood TSH and T3 or T4 were found with maternal blood levels of organophosphate DAP metabolites throughout pregnancy (374). Similar to the relationship with maternal DDE and HCB, cord blood levels of these chemicals were also associated with reduced levels of total T3 and T4 (435).

The association of cord blood EDCs has also extended to cord blood levels of steroidal hormones, with methyl and propyl paraben associated with reduced T levels (438), BPA with increased E2 levels (294), phthalate metabolites with reduced levels of cortisol and cortisone, and glucocorticoid/adrenal androgen ratio, and increased DHEA and DHEA/androstenedione ratio (439) and PCB with decreased maternal T and increases in maternal E2 levels (440). The effects of PFAS appear to be metabolite specific, with PFOS associated with increased maternal DHEA and cortisol, whereas PFOA is associated with decreased DHEA levels in cord blood (441). Importantly, EDC effects in newborns showed sex specificity, with agrochemicals CHD, HCB, heptachlor epoxide, Mirex, and toxaphenes associated with reduced cord blood levels of T, cortisol, cortisone, and prolactin, as well as with increased DHEA and follicle-stimulating hormone among boys (442), as opposed to DDT being associated with decreased cord blood DHEA and increased cortisol levels among girls (442). These studies indicated EDCs altered fetal hormonal milieu potentially in a sex-specific manner, a crucial aspect not investigated in many studies.

Metabolomics

In the growing fetus there is a great demand for the metabolites that serve as an energy source and building blocks to aid in rapid tissue differentiation and metabolism (445). Therefore, factors that regulates fetal metabolites themselves are under tight regulation. For example, fetal glucose metabolism has been found to be dependent not only on maternal glucose status but also on fetal glucose and insulin secretion (446). Similarly, the concentration of leptin in neonatal cord blood has been suggested to be an indicator of fetal adiposity (447). Evidence relative to the impact of EDCs on fetal metabolites is rather limiting. Limited evidence has indicated a potential impact of EDCs on cord blood levels of glucose and energy regulatory molecules (see Table 3). These include associations of 1) BPA and phthalate metabolites MiBP, MnBP, and DEHP in maternal first-trimester serum with reduced levels of leptin; 2) maternal first-trimester circulating BPA levels with increased HMW adiponectin levels (448); and 3) maternal first-trimester urinary DAP with increased insulin (449) in neonates. Cord blood levels of EDCs have also been shown to be related to decreased cord blood levels of metabolic factors such as insulin with increased PCB153 (449) as opposed to increased fatty acid metabolites 17-methylstearate, laurate (12:0), and 4-vinylphenol sulfate with inorganic As (400). Of importance, cord blood level of the persistent organic pollutant DDE showed a sexually dimorphic association with low adiponectin and insulin levels in cord blood of girls but not boys (449). Although the changes in fetal metabolome are apparent for some commonly studied EDCs, similar information for many other EDCs is lacking, opening a fertile area for future investigations.

Microbiome

Although the mechanisms are poorly understood, there is general agreement that microbial colonization of the fetal gut occurs in utero (450, 451). The presence of bacteria in amniotic fluid without premature rupture of membranes supports transmission of bacteria from the mother and bacterial colonization in utero (408, 452-454). Although EDC effects on the microbiome profile in the fetus are not known, the altered fetal microbiome, analyzed through detection in the meconium, has been found to be associated with gestational complications like GDM (455), and linked with adverse birth outcomes like PTB (454, 456) and long-term health of the offspring (457). Because maternal and fetal/neonatal presence of EDCs has been linked with such adverse gestational and fetal outcomes, part of the effects may involve an altered fetal microbiome.

Epigenetics

Epigenetic alterations involve heritable changes to the genome that are induced through modifications to DNA or histones and expression of noncoding RNAs (458). The DNA modification chiefly occurs through methylation, leading mostly to silencing of gene expression (459, 460). On the other hand, histones may be modified through methylation, acetylation, phosphorylation, sumoylation, and ubiquitination and have permissive or repressive effects on gene expression (461). Noncoding RNAs such as micro (miRNA), small RNA, long RNA (lncRNA), and circular RNA have a role in posttranscriptional regulation of gene expression (462, 463). These various epigenetic mechanisms affect gene function through which EDCs mediate short- and long-term health outcomes (13, 464-466). Evidence supports the association of EDCs in maternal and cord blood with epigenetic alterations in the fetal epigenome (467) (see Table 3). These include the association of 1) increased methylation of long interspersed nuclear elements (LINE) 1 and p16 gene with inorganic As (468); 2) lower Alu methylation with DDT/DDE (468), and MEP (469); 3) decreased TNF methylation with PBDE levels (429); 4) decreased methylation of LINE1, insulin-like growth factor 2 (IGF2) and PPARα with MCPP; and 5) decreased PPARα methylation with presence of MBzP and DEHP (470) in the maternal milieu. Importantly, some of the EDC impact has been found to be sex specific with maternal urinary As (471, 472) and Cd (473) showing increased methylation changes in boys as opposed to being decreased in girls. Similarly, presence of maternal first-trimester urinary BPA was associated with decreased methylation of IGF2, and PPARα in cord blood leukocytes only in female neonates (470). Some studies have associated fetal or newborn levels of EDCs with epigenetic outcomes. For instance, BPA in the human fetal liver showed increased methylation in CpG islands and decreased methylation in CpG shores, shelves, and repetitive regions in fetal liver when using an epigenome-wide approach (474). Total and methyl Hg in cord blood were also found to be associated with differential methylation of genes ANGPT2, PRPF18, FOXD2, and TCEANC2 (475). These studies provide evidence that sex-specific associations underlie maternal or fetal presence of EDC with changes in epigenetic biomarkers. Similar studies addressing the impact of EDCs on epigenetic alterations in key genes controlling maternal and fetal function are very much needed to help develop targeted interventions.

The Effect of Endocrine-Disrupting Chemicals on the Placenta

The placenta, a transient organ, serves as an interface between maternal and fetal vascular beds to aid in nutrient and waste exchange and support of in utero life. It acts as a buffer to protect the fetus from immunologic, endocrine and environmental insults and participates in cholesterol and steroid biosynthesis (476). The placenta is also a dynamic organ with the ability to adapt to maternal and fetal needs, and the large size and surface area of the placenta along with the presence of transporters and metabolizing proteins aid in this function (477). Disruptions in placental homeostasis could affect the fetal growth trajectory, leading to IUGR and SGA babies (478). Environmental exposure to EDCs has been shown to disrupt placental function (234, 479). Some EDCs could cross the placental barrier directly affecting the fetus while others could induce changes in the biosynthesis of placental hormones and function and thus indirectly affect the fetal developmental trajectory. Placental dysfunctions could also be facilitated via reduced nutrient availability, diminished vascular supply, and/or compromised hormonal, inflammatory, oxidant and metabolic states all of which could affect the maternal and fetal milieu. Although the placenta begins to form around 10 days into pregnancy and is important for maintenance of the pregnancy throughout, information on EDC impact on early pregnancy placental changes are not available in humans, with the majority of epidemiological studies being confined to placentae obtained at the time of delivery.

Placental structure and size

With the presence of syncytiotrophoblast, cytotrophoblast, and fetal endothelial cells, the human placenta acts as a physical barrier to separate fetal blood from the intravillous space that contains the maternal blood through which transfer of nutrients, gas, and other molecules are facilitated (480). The thickness of the placenta is one of the determinants of the barrier function that affects the permeability and bi-directional transfer of substances between the mother and fetus. At term, the placenta is discoid shaped, of 15 to 25cm in diameter, with an approximately 3-cm thickness and weighing about 500 to 600 g. The examination of the placenta soon after the delivery provides valuable information because altered placental morphology is associated with fetal and maternal abnormalities (481). Currently, placental morphology is assessed at delivery through gross examination and histological techniques, and there is a need for advanced imaging to facilitate early detection of placental abnormalities. Although studies addressing the association of changes in placental morphology with EDCs are limited, available evidence has indicated an association between increased placental width with maternal first-trimester urinary MBP as well as increased placental thickness with second trimester urinary MMP, MBP, MEOHP, and MEHHP, and MBP and third-trimester urinary MEH (482). High concentrations of placental Cd were also associated with smaller volumes of fetal capillaries and capillary surface-to-volume ratio, increased maternal blood space relative to placental volume, and a thickened trophoblast component of the villous membrane (483). One study indicated PBDE congeners in the umbilical cord blood at birth was associated with reduced placental length, breadth and surface area (484). These findings, along with reports that the presence of PBDE decreased cytotrophoblast differentiation marker integrin α-1 and increased interstitial/endovascular uterine invasion protein vascular endothelial-cadherin (485), suggest that EDC affects placental formation and function involving placental structural changes.

Placental efficiency

Optimal fetal growth depends on adequate provision of nutrients by the placenta. Any placental compromise leading to inadequate nutrient supply would result in the fetus failing to achieve its growth potential. A positive correlation between placental weight and birth weight suggesting that the size of the placenta dictates the size of the fetus is well established from studies in animals (486) and humans (487). The ratio between the birth weight and placental weight serves as an index of placental efficiency. A reduction in this ratio indicates the inability of the placenta to adapt its nutrient transfer capacity to compensate and overcome growth restriction (488, 489). Only a few studies have capitalized on this index with respect to EDC impact. One such study has found the late-gestational presence of phenols triclosan and benzophenone-3, and phthalates MCNP and MCOP to be associated with a reduced birth weight to placental weight ratio (490). Considering the value of this index in determining immediate and long-term health outcomes (202), future studies need to incorporate this easily obtainable measure in their investigations.

Mediators of placental function

Functional homeostasis of the placenta is essential for efficient fetal support. Disruptions in placental function may be mediated via changes in inflammatory cascade, oxidative stress, and hormonal support, with many of these changes involving epigenetic alterations. Although most of these data come from animal models, placental explants, and cell lines (491-494), limited studies provide evidence (Table 4) pointing to the EDC-associated changes in mediators of placental function in humans, and these are discussed below. The presence of EDC in the maternal compartment has been found to be associated with changes in placental inflammatory cytokines reflective of its state of inflammation. The association of maternal urinary phthalates with placental cytokine expression appear to be EDC metabolite and sex specific (see Table 4). For instance, maternal first-trimester urinary MBP was associated with increased IL-1B, IL-6, and CRP in placentae associated with male fetuses and increased IL-6, CRP, CCL2, IL-8, IL-10, and CD68 in those associated with female fetuses (495). In contrast, maternal first-trimester urinary MEOHP was associated with decreased CRP, CCL2, CD68 mRNA only in female placentae, whereas MBzP was linked to increased TNF, CCL2, and CD68 mRNA in male placentae collected at term (495). The association with metals depended on the time of the pregnancy, with maternal first-trimester urinary levels of As linked with decreased in IL-1B, TNF, IFNG (355) and Tl with increased CD68 and decreased IL-4 and CD206 (496). While second-trimester levels of As were associated with decreased CCL2, IL-6 and TNF (497) third-trimester Tl was associated with increased placental TNF, IL-6, and CD68 expression (496).

Table 4.

Early and late gestational presence of endocrine-disrupting chemicals and their associations with placental changes

EDC Classes	Inflammatory and oxidative state		Hormonal and metabolomic changes		Epigenetic changes
	Early	Mid to late /term	Early	Mid to late /term	Early	Mid to late /term
Metals		Maternal TM1 As: ↑8-oxoguanine, IL1B, TNF, IFNG (355) Maternal TM1 Tl: ↑ CD68; ↓ IL4 AND CD206 (496) Maternal TM2 Co: ↓CCL2, IL6 and TNF (497) Placental Mn: CD68 colocalication with Mn (499) Placental Cd: ↑ Di- and nitrotyrosine; ↓antioxidants (34) Maternal TM3 As: ↑8-oxoguanine and IL1B (355) Maternal TM3 Tl: ↑ TNF, IL6 and CD68 (496)		Placental Cd: ↓ placental progesterone production (500); ↓ leptin (502) Maternal TM3 As:↑leptin (355)	Maternal urine Pb: ↓ methylation at 9 CpGs (513)	Placental Cd: 17 differentially methylated CpG sites with ↑ TNFAIP2, EXOC3L4, GAS7, SREBF1, ACOT7, and RORA (506) ↑ miR-1537 and ↓ let-7 family members (514) Placental Hg: ↓ let-7 family members (514)
Parabens						∑parabens ↑miR-128 (512)
PAHs		Placental benzopyrene: ↑ oxidant malondialdehyde and ↓ antioxidant glutathione (216)		Cord-blood PAHs: ↑IGF1 and IGFBP3 (501)
Phytoestrogens
Bisphenols						TM2 aborted fetus BPA: ↑ miR-146a and 33 other miRNAs (511) Placenta BPA ↑ LINE1 methylation (515) Maternal ∑phenols: ↑miR-128; ↓miR-142, miR-15a-5p (512)
Phthalates		Maternal MBP: ↑mRNA expression of IL1B, IL6, and CRP (♂); ↑IL6, CRP, CCL2, IL8, IL10, and CD68 (♀) (495) Maternal MEOHP: ↓ CRP, CCL2, CD68 mRNA (♀) (495) Maternal MBzP: ↑TNF, CCL2 and CD68 mRNA (♂) (495)	Maternal MEHHP, MEOHP, MECPP, ΣDEHP: ↓PlGF (379)	Maternal MnBP: U-shaped dose–response with steroidogenic genes (322) Maternal MEHHP, MEOHP, MECPP, ΣDEHP: ↓PlGF (379)	Terminated pregnancy ∑phthalates: 2214 differentially methylated single CpG sites; 282 differentially methylated regions (518)	MEOHP or MEP: ↓ IGF2DMR0 methylation (♂ and ♀) (509) Maternal ∑LMP (MiBP, MnBP, MEP): ↑miR-185 (512) Maternal MECPP, MEHHP, MEHP: ↓IGF2DMR0 methylation (♀) (509) Maternal MCNP and MHiBP: ↑ placental lncRNAs (510) Maternal DEHP: ↓ LINE1 methylation (516)
Organohalogens				Placental PBDE: ↓ T4 (examined in males only) (163) Cord PBDE: ↑IGF1 and IGFBP3 (501)		Placental PBDE: ↑ global DNA methylation (507) Placental PCB: ↑ H19 methylation (507) Maternal BDE ↑ DIO3 methylation (♀) (507) Maternal POP: 214 differentially methylated CpG sites (517) Maternal PBDE ↑ miR-188-5p ↓ let-7c; PCB ↑ miR-1537 (514)
Agro- chemicals				Placental methoxychlor: ↓ free T3 (♂ examined) (163)		Maternal DDE: ↑ DIO3 methylation (♀) (508) Maternal DDT: ↑ MCT8 methylation (♂) (508)
Antifouling agents				Placental tributyltin and ∑organotin: ↓ free T3 (♂ examined) (163)
Personal care products	see above sections for effects of parabens, bisphenols and phthalates
Pharmaceuticals	see above sections for effects of bisphenols and phthalates

Abbreviations: ↑, increase; ↓, decrease; ≠, no association; ∑, sum; ♂, male; ♀, female; CRP, C-reactive protein; DDE, diphenyl dichloro ethylene; DEHP, phthalate di(2-ethylhexyl)-phthalate; Early, first trimester; EDC, endocrine-disrupting chemical; IFNG, interferon γ; IGF1, insulin-like growth factor; IL, interleukin; mid to late, second and third trimester; mRNA, messenger RNA; miRNA, microRNA; PAHs, polyaromatic hydrocarbons; PBDE, polybrominated diphenyl ether; PCB, polychlorinated biphenyl; T3, 3,5,3′-triiodothyronine; T4, thyroxine; TM, trimester; TNF, tumor necrosis factor.

As is the case with inflammatory markers, EDCs were found to also be associated with changes in placental oxidative stress status. Causal links for such association come from many in vitro studies and animal models (498). Limited studies in human have indicated 1) first- and third-trimester maternal urinary levels of As are associated with increased oxidative marker 8-oxoguanine (355); 2) placental presence of Cr are associated with increased dityrosine and nitrotyrosine, with the increase higher in placentae involving male than female fetuses and a concurrent reduction in antioxidant proteins also showing sexual dimorphism with decreased activities of GPX and catalase in male placentae, and decreased GPX and increased SOD activities among female placentae (34); and 3) placental benzopyrene is associated with increased oxidant malondialdehyde and decreased antioxidant glutathione (216).

Because the placenta is also an endocrine organ, EDCs can also affect placental hormonal production (see Table 4). Evidence to date point to 1) reduced placental progesterone production with presence of Cd in the placenta (500); 2) a nonmonotonic U-shaped dose–response relationship with expression of steroidogenic genes aromatase, 17β-hydroxysteroid dehydrogenase, cholesterol side chain cleavage, and Cytochrome P450 1B1 in term placenta with presence of maternal third-trimester urinary MnBP (322); 3) increased placental leptin with third-trimester maternal urinary As (355); 4) increased placental IGF1 and IGFBP3 expression with cumulative cord blood levels of PAH and PBDEs (499); and 5) changes in placental thyroid hormones linked with placental persistent organic pollutants (163).

Epigenetic mechanisms are involved in the regulation of gene expression during development, and the placenta also undergoes dynamic epigenomic changes throughout pregnancy. Therefore, alterations in these placental epigenetic marks have been found to be associated with placental defects affecting the mother and fetus and long-term health of offspring (503, 504). Because of this, assessment of the placental epigenome is also important for understanding the developmental impact of EDCs (505). A limitation of placental studies in humans is that, although the placenta undergoes dynamic epigenetic changes throughout pregnancy, most observations have been limited to term, and early changes have been missed. Numerous reports have provided evidence in support of involvement of epigenetic alterations relative to EDC effects on the placenta (see Table 4). For instance, presence of Cd in term placenta was associated with about 17 differentially methylated CpG sites and increased expression of genes TNFAIP2, EXOC3L4, GAS7, SREBF1, ACOT7, and RORA (504). Similarly, the organohalogen PBDE present in placenta was associated with a global increase in DNA methylation in contrast to PCB being associated with increased methylation at the H19 locus (507). Maternal levels of EDCs have also shown to be associated with placental DNA methylation. Evidence supporting this premise includes a sex-specific association of maternal serum BDE and DDE at delivery with increased methylation of thyroid hormone-related gene deiodinase type 3 (DIO3) in females and the association of DDT with methylation of monocarboxylate transporter 8 in males (508). Other studies have reported that a sex-specific association of methylation of a regulatory region in IGF2 in the placenta depended on the phthalate metabolite, with maternal first-trimester urinary MECPP, MEHHP, MEHP linked with a decreased IGF2 methylation in females and MEOHP and MEP with a decrease in both sexes (509). EDCs have also affected the expression of noncoding RNAs in the placenta with maternal term urinary MCNP and MiBP showing an association with increased expression of lncRNAs (510). In one study, the presence of BPA in an aborted fetus at the second trimester was found to be associated with increased expression of 33 miRNAs in the placenta (511). Assessment of the cumulative effects of maternal first-trimester urinary EDCs showed that parabens in aggregate were associated with an increased placental miR-128, LMW phthalates (MiBP, MnBP, MEP) collectively with increased miR-185, and phenols altogether with increased miR-128 and decreased miR-142 and miR-15a-5p (512). These observations indicate that, as in the maternal and fetal compartment, altered placental inflammatory, oxidative, endocrine, and metabolic states and associated epigenetic modifications integrate to mediate EDC action and affect gestational and birth outcomes.

Long-term Consequences of Gestational Endocrine-Disrupting Chemical Exposure

The landmark studies with the Dutch Hunger Winter birth cohort have shown that the impact on birth weight of the offspring varies depending on the window of exposure to famine and undernutrition during pregnancy (519). The observations by Barker and colleagues that children with low birth are at high risk of developing coronary diseases led to the proposal of fetal origin of disease hypothesis (6). Later, the recognition that adverse exposures expand beyond just nutrient restriction and include environmental exposure to EDCs culminated in the DOHaD hypothesis (7). The impact of EDCs on the maternal and fetal milieu on birth outcomes, a risk factor for development of adult-onset noncommunicable diseases (5), is discussed earlier and is summarized in Table 2. Possible long-term consequences of maternal/fetal exposure to EDCs based mostly on animal studies have been extensively reviewed (16, 520-526). Observations from children born during past environmental disasters and emerging evidence from cohort-based studies that have measures of EDC during pregnancy and available outcomes in children have provided evidence that developmental EDC exposure can lead to adverse health consequences in children. Information from past disasters supportive of gestational programming by EDCs include exposure to i) Hg in Japan’s Minamata Bay that led to severe brain dysfunction in their children (527); ii) PCB in rice oil in Japan and Taiwan that resulted in children with poor cognitive development (528); and iii) dioxin from a chemical factory explosion in Seveso, Italy, that was associated with development of reproductive and cardiometabolic disorders and increased risk for cancer among children and decreased sperm count in their grandsons (529). Prospective birth cohort studies have started to link gestational presence of EDCs with neurobehavioral, immunologic, metabolic, and reproductive disruptions. Evidence linking maternal presence of EDCs with neurodevelopment include (1) a positive association of maternal urinary BPA concentrations at 16 weeks of gestation with externalizing (aggression and hyperactivity) behaviors in children at age 2 years (530), and anxious and depressed behavior at age 3 years (531) with this association strongest among girls; (2) an association of maternal third-trimester urinary HMW phthalates with sex-specific behavioral changes with better orientation and motor scores among boys and poorer orientation and quality of alertness among girls (532), (3) a linkage of LMW phthalates with more self-reported hyperactivity, attention problems extending to age 16 years in the CHAMCOS cohort (533); (4) an association of maternal PBDEs at 16 weeks of gestation with decreased PFASs, PFOA, and PFNA, with increased reading scores among 8-year-old children (534); and (5) an association of higher levels of gestational urinary DEHP and DBP metabolites in Korean mothers, with lower mental and physical developmental scores among infants (535).

Relative to offspring metabolic outcomes, first-trimester BPA levels were found to be associated with decreases in girls and increases in boys of body mass index (BMI) and adiposity measures at age 4 years (536). Association of PFAS was metabolite specific, with gestational presence of PFOA at 16 weeks of gestation being associated with an increase in BMI from age 4 weeks to 12 years, a risk factor for adult obesity and cardiometabolic disease, as opposed to PFOS and PFHxS exposure being associated with lower BMI during the first 12 years of child’s life (537). Importantly, the associations with PFAS appeared to be sex dependent, with prenatal exposure to PFASs linked with small increases in adiposity measurements in midchildhood among girls only (538). For phthalates, the associations with adiposity measures varied not only by phthalate metabolite and sex but also by timing of exposure. First-trimester presence of MIBP was associated with adiposity measures such as increase in skinfold thickness, BMI for age, and waist circumference among girls and diametrically opposite in boys in the presence of second-trimester MBzP (539).

At the reproductive level, gestational phthalates in a metabolite-specific manner were linked with changes in peripubertal serum T levels in girls (540). Specifically, MEP across pregnancy, MiBP in the second trimester and MBzP in the third trimester, were associated with higher T levels, while third-trimester DEHP was linked with higher DHEA-S in girls (540). Second-trimester levels of BPA were also linked with higher serum T in peripubertal girls. This same study also found MEHP across pregnancy to be associated with lower odds of having a Tanner stage greater than 1 for breast development while MEHP in the third trimester was associated with higher odds of having a Tanner Stage >1 for pubic hair development (540). Detailed information from a large diverse cohort on the impact of gestational EDC exposure on child health outcomes will be forthcoming through the National Institutes of Health (NIH) initiative Environmental Influences on Child Health Outcomes Program. This program with nearly 50 cohorts spread across the United States will relate environmental chemical exposure from more than 50 000 mother-child dyads (541) on various childhood outcomes.

The impact of gestational EDC exposure on long-term health outcomes extending into adulthood are not available at the present time. A causal link between EDC exposure and long-term outcome can only be surmised from findings in children born to pregnant women treated with diethylstilbestrol (DES), a synthetic estrogen widely used before 1971 to prevent miscarriage. Daughters of mothers treated with DES during pregnancy were associated with increased odds for infertility, spontaneous abortion, ectopic pregnancy, PE, still birth, preterm delivery, early menopause, cervical intraepithelial neoplasia, and breast cancer (542, 543). In sons born from DES-treated pregnancy, early effects on reproductive tract malformations such as cryptorchidism were well known; however, the long-term effects were inconclusive (544). Male and female offspring were also characterized by varying degrees of psychological effects including depression and anxiety behavior (545). In addition, transgenerational transmission of traits that have been well established in animal models (546-548) are beginning to emerge among grandchildren of women exposed to DES or dioxin during pregnancy. These include significantly elevated odds for attention-deficit/hyperactivity disorder among grandchildren of DES exposed pregnant women (542) and decreased sex ratio and elevated thyroidal hormones among grandchildren of Seveso women (529). These findings indicate that many of the traits induced by in utero exposure to EDCs not only have short-term and long-term effects on the offspring but have the potential to be passed on to subsequent generations.

Interaction of Endocrine-Disrupting Chemicals With Genetics, Diet, Stress, and Lifestyle

As discussed in earlier sections, pregnant women are exposed to a large number of EDCs that affect the maternal, placental, and fetal milieu either by acting directly as hormone agonists or antagonists or indirectly influencing inflammatory and oxidative status, hormonal, metabolomics, and microbial factors. In addition to the maternal, placental, and fetal milieu interacting with each other to dictate the effects of EDCs on pregnancy outcomes, other factors such as diet, stress, lifestyle, and their genetic makeup can modulate the impact of EDCs.

With diet being a major source of exposure to EDCs (549), many of the persistent organic pollutants because of their lipophilic nature can bioaccumulate (550), leading to biomagnification in animals including humans who are higher up in the food chain (550, 551). Results from a longitudinal pregnancy study in the United Kingdom demonstrated that pregnant mothers who ate meat have higher odds than vegetarian mothers for development of hypospadias, with the odds reduced even further when mothers only consumed organic food (552). Such findings suggested that consuming food exposed to chemical fertilizer and pesticides via nonorganic food were likely associated with higher incidences of hypospadias. Studies in animal models and prospective studies in humans have indicated that a well-balanced diet containing antioxidants, long-chain ω 3 polyunsaturated fatty acids, and flavonoids could provide protection from adverse effects of EDCs by reducing oxidative stress and inflammation (553-556) and odds of IUGR in animal models and humans (557), outcomes associated with EDC exposure (see Table 2). Similarly, diets rich in antioxidants and anti-inflammatory supplements have been shown to reduce maternal urinary Cd-associated increase in circulating inflammatory biomarkers (CRP, γ-glutamyltransferase [GGT], and alkaline phosphatase) (558). The protection from maternal BPA-associated implantation failure and reduction in live birth rate by soy dietary supplementation (559) as well as reduction in incidences of hypospadias among offspring of women with occupational exposure to phthalate containing hair spray products (560), and increased odds of live birth among mothers exposed to air pollution by folate supplementation (561) have provided additional support for diet-EDC interaction. Considering that part of the effects of endocrine disruptors involved epigenetic modulation, the classic demonstration that dietary supplementation of methyl donors such as folic acid antagonized the shift in coat color in the agouti mouse induced by BPA via DNA hypomethylation (562), provides an avenue for such modulatory effects of EDC.

As with diet, exposure to psychosocial stress is independently linked with adverse pregnancy and birth outcomes such as gestational hypertension, GDM, abortions, PTB, and low birth weight (563-565), outcomes also affected by exposure to EDCs. Because humans are exposed to psychosocial stress due to racial disparities and discrimination, poverty, and depression or anxiety in parallel, the interaction between these 2 variables is an area of concern (566, 567). The recognized impact of psychosocial stress and EDCs on glucocorticoids during pregnancy (see Table 3) has provided a common mediator for these 2 variables to interact at the level of the hypothalamus-pituitary-adrenal axis. For example, modification of EDC effects by maternal stress was demonstrated by a meta-analysis that showed exposure both to high levels of stress and EDCs (as a result of smoking and traffic pollution) were independently associated with reduced birth weight, but the combination of the 2 has significant negative outcome on birth weight (568). Another study demonstrated that the relationship between EDC and gestational length was stronger among women with more adverse life events (569). In this study, maternal urinary triclocarban and BPS across pregnancy were strongly linked with shorter gestational length as opposed to methyl- and propyl-parabens, which were strongly linked with increased gestational length in mothers with stress-inducing negative life events during pregnancy.

While lifestyle choices such as use of personal care products, smoking, eating fast food, canned food, and high-caloric diet vs organic and vegetarian diet can influence the amount of EDC exposure, other lifestyle choices like exercise vs sedentariness, smoking, and alcohol and/or drug usage can also dictate the outcomes of EDC action. A sedentary lifestyle during pregnancy was found to be associated with negative pregnancy outcomes both for mother and child, including gestational weight gain, hypertension, and altered birth weight (570), outcomes also associated independently with EDC exposure. The reported additive effect of a sedentary lifestyle with the urinary phthalate metabolite MEOHP on circulating levels of progesterone, follicle-stimulating hormone, and luteinizing hormone (571) suggest an interaction between sedentary lifestyle and EDC exposure during pregnancy. To what extent this is related to sedentary lifestyle contributing to excessive weight gain and obesity (570) needs to be ascertained. The relationship between maternal second-trimester circulating PBDE with second- and third-trimester inflammatory cytokine TNF was much stronger among obese women compared to nonobese women when compared with PBDE alone (352). This suggests disease status, diet, or lifestyle, modification, which are potential contributors to the obesity and consequent changes in the mother’s metabolic milieu may modulate EDC action. Likewise, the smoking status of mothers during pregnancy was shown to modify EDC association with birth outcomes, with an increase in odds of SGA associated with HCB and PFOS exposure found only in mothers who smoke (265).

A challenging public health problem in the United States is the racial/ethnic disparities in pregnancy outcome, with infant mortality and low birth weight rates shown to be higher among African American women compared with White women (572, 573) and associated economic burden (574). The disproportionate exposure of African American women to environmental EDCs (575-577) appear to be one contributing factor for this disparity in pregnancy outcomes. In addition, African American communities were found more likely to experience widespread poverty, have lower high school graduation rates, high unemployment rates, food insecurity, increased access to poor-quality foods such as canned and fast foods, and less access to physical activity and health care (578-581). As discussed earlier, considering these changes are associated with changes in diet and lifestyle and increase in psychosocial stress, they have the potential to interact with EDCs to modulate their effects.

The genotype or genetic make-up is a major contributor relative to susceptibility of an individual to EDC exposure. Genome mapping shows that the genome is 99.9% identical among unrelated individuals (582). The main source of the variation amongst individuals are polymorphisms of which the most common are single-nucleotide variations (SNVs, formerly single-nucleotide polymorphisms [SNPs), where a single nucleotide in the genome sequence is altered (583). These variations can decree the susceptibility of the individual for the EDC through an altered target receptor that can either reduce or enhance binding of the EDC or through changes in xenobiotic metabolizing enzymes that dictate bioavailability by neutralization, biotransformation, or potentiation (584). That genetic susceptibility can also be modified by environmental factors including diet and lifestyle factors has led to the concept of gene-environment interaction (GxE), which forms the basis of many developmental disorders (585) including alterations in pregnancy milieu. This GxE instills differential susceptibility depending on the genes that are involved in responsivity to environmental states. As such these genes can increase the risk for adverse pregnancy outcomes in a negative environment, or alleviate the risks in positive environments (586). In the Seveso Women’s Health Study, women exposed to dioxin found neither an SNV in AHR nor dioxin exposure was independently associated with birth weight, but considered together the association was significantly stronger with birth weight (587), thus supporting the GxE interaction in modifying the susceptibility to EDC exposures. These findings highlight the complexity associated with the risk assessment of EDC exposure and emphasize the need to consider genetics and lifestyle factors to understand the true impact of EDCs on pregnancy outcomes, an aspect not considered in most studies.

Conclusions and Future Directions

The evidence discussed thus far indicates that EDCs across all classes of chemicals have the potential to impinge on the pregnancy milieu leading to adverse pregnancy, birth, and long-term health outcomes. As discussed in detail in earlier sections, the actions of EDCs may be direct or facilitated via altered maternal and fetal placental homeostasis and function and involve alterations in inflammatory, oxidative, hormonal, metabolomic, microbiomic, and epigenomic status subject to modulation by genetic makeup and environmental and lifestyle factors. The limitations of this field and the directions future research (see Fig. 2) should take are discussed as follows.

Figure 2.

Schematic identifying the key gaps and challenges in integrating the knowledge to understand the complex processes and outcomes affected by endocrine-disrupting chemical exposure during pregnancy. These challenges may be potentially overcome through sophisticated statistical modelling involving data science, machine learning and artificial intelligence to ultimately develop effective risk mitigation strategies.

Need for Determining Internal Exposure Levels

For several EDCs, the biomonitoring assessment of exposure is based on measures of EDC metabolites in early morning or 24 hour voids of urine (588). Questions have been raised for certain EDCs on the validity of 24-hour urine measure because of the potential for day-to-day variability in exposure (589, 590). In view of differences in clearance rates and the potential for bioaccumulation, urinary measures do not provide actual internal levels and patterns of EDC exposures. This is important even for EDCs that clear quickly in view of rapid signaling mechanisms that these chemicals can operate through (589, 590). For instance, repetitive frequent exposures to an EDC such as BPA were effective in inducing effects at the cellular level, in spite of their rapid clearance (591). In light of this understanding, there is a clear need to develop techniques to accurately estimate internal exposure levels of EDCs.

Need for Longitudinal Measures of Endocrine-Disrupting Chemicals

Another limitation of studies addressing human exposure to EDCs during pregnancy are snapshot, single time-point assessments. Humans, however, are continuously exposed to EDCs that have the potential to bioaccumulate in tissues and be released back into circulation at a later time. Furthermore, the susceptible developmental windows differ throughout pregnancy depending on the parameter being assessed. For instance, assessment of ambient air pollution across pregnancy has narrowed down susceptible developmental windows for PTB associated with air pollution to be the first 2 trimesters of pregnancy (592). A few studies have determined BPA, phthalate, triclosan and metals at more than one-time point during pregnancy (see Table 2). One such study, measuring urinary levels of phthalate metabolites at 4 time points during pregnancy, showed differing exposure levels across gestation (206). Another aspect to consider is that duration of EDC exposure varies with the EDC half-life (593). When urine samples provide the main solution matrix for EDC assessments, EDCs with a longer half-life (eg, persistent organic pollutants, PFAS) have greater measurement reliability, whereas EDCs with a short half-life (eg, PAH, BPA), show greater variability among individuals and thus low reproducibility, even when urine collections are taken from the same individual (589, 590). Reliable techniques for repeated assessment of EDCs are therefore required spanning the entire pregnancy. When considering longitudinal assessment of EDC exposure, and when feasible, consideration needs to be given to preconception exposure to EDCs, as gamete exposure is also germane. In fact, exposure to EDCs during preconception has been found to be associated with poor pregnancy and birth outcome (594), suggesting EDC susceptible developmental windows begin well before pregnancy.

Need for Establishment of Endocrine-Disrupting Chemical Dose-Response Relationships

Traditionally, risk assessments of chemicals are based on establishing dose-response relationships to define maximal-effect level (Emax; a toxicant dose beyond which any increase will not increase the response), no observed adverse effect level (NOAEL), and/or the lowest observed adverse effect level. EDCs, like many hormones, however, follow nonmonotonic dose-response relationships (595). In today’s regulatory environment, NOAEL is considered a conservative default threshold below which an EDC is not expected to induce adverse effects (596). As discussed earlier for gestational and birth outcomes, inverted–U-shape profiles, with adverse effects at the intermediate exposure level and reduced or no effect observed at low- and high-exposure levels, and U-shaped profiles, with the adverse effects at lower and higher EDC exposure levels, both have been reported (318, 593). Standard approaches of dose testing to establish the NOAEL doses are therefore not valid for EDCs, and future studies should assess the nonmonotonicity of each dose relationship on adverse gestational outcomes.

Need for Assessment of Cumulative Endocrine-Disrupting Chemical Burden and Impact

As discussed earlier, most studies assess the impact of a single EDC or EDC class, but humans are exposed to multiple EDCs with differing chemical classifications. Considering the varying signaling mechanisms and potential for cross talk through common and divergent target receptors affected (see Fig. 1), there is a clear need to relate outcomes to the mixture of EDCs to which one is exposed. Although studies of this nature are emerging, many focus on a single class of chemicals, such as mixtures of phthalate metabolites. Because humans are exposed to multiple EDC classes in parallel, expansion of such studies to “real-life” EDC mixtures encompassing several classes is needed, combined with in vitro and in vivo models to establish true MRA. Such “bottom-up” approaches are necessary for EDC MRA, as current legislative efforts in regulating EDC address regulatory issues, one chemical at a time (597). In developing legislation, it is important to recognize that human EDC exposure varies geographically. For example, a Danish study comparing rural vs urban distribution of common EDCs in pairs of mothers and children found no difference in phthalate presence, but detected more BP3 and parabens among urban mothers and children (598), indicating a need for geographical considerations in the analysis of cumulative exposure burden. Alternate matrices such as hair, nails, meconium, and shed deciduous teeth (in children) should be capitalized on to gain an understanding of cumulative EDC burden.

Need to Establish Sex-Specific Measures and Outcomes

It is becoming increasingly apparent that EDC effects differ between the sexes (599). The finding that efficiency of the placental transfer of metals Ti and Ag were significantly higher among male than female human fetuses (600) indicates fetal sex-related differences in placental transfer of EDC may also contribute to sex-specific differences in detrimental gestational and birth outcomes. In addition, the placenta, which is mostly of fetal origin, also manifests sexually dimorphic traits (601). Only a few human studies, however, have considered fetal sex when examining the placenta as a determining factor limiting the impact of EDCs (493). These studies have shown a sex-specific impact of EDCs on placental epigenetics (602) or maternal/neonatal thyroid hormone levels (603). Because of the differences in how males and females respond to stressors, including chemicals, since 2014 the NIH has implemented policies to encourage the understanding of sex-specific actions and effects on health outcomes (604), an aspect that should be incorporated into measures of EDCs and their effects.

Need to Consider Genetic, Environmental and Life Style Factors in Modulating Susceptibility to Endocrine-Disrupting Chemical Exposure

Considering the difference in genetic makeup and the burden of exposures to chemical and non-chemical stressors between people from different racial/ethnic background and their influence on susceptibility to EDCs, population-based studies should account for such differences when assessing EDC impact. In support of this premise, homozygosity in a specific estrogen receptor α haplotype with SNV among cryptorchid individuals showed heightened susceptibility to estrogenic EDCs (605). As discussed earlier, factors that can modulate susceptibility to EDCs include diet, physical activity, smoking, and medications. For instance, lack of exercise and/or a high-caloric diet intake can promote obesity leading to sequestration of lipophilic EDCs in fat depots resulting in increased exposure and susceptibility to EDCs (606). Considering many of these factors pose risks themselves, relative to the outcomes of EDC impact, epidemiological studies need to account for such variables and their potential interactions with EDCs. With more women having children later in their life (607), and one study reporting increased exposure levels of EDCs in older women (608), another fruitful area of research involves maternal age and its influence on the impact of EDCs.

Need to Develop Approaches to Understand the Placental Impact of Endocrine-Disrupting Chemicals

A major limitation relates to the lack of knowledge relative to early exposure level and effect of EDCs on the placenta. Development of the placenta occurs before that of the fetus to meet the demands of the growing fetus, and its vasculature is pliable to adapt to environmental conditions and to changes in maternal blood supply; because of this, the placenta undergoes considerable remodeling throughout pregnancy (609). The effect of EDCs on these variables cannot be assessed at delivery alone. Where possible, birthweight to placental weight ratio should be capitalized on as a surrogate to gain an understanding of EDC impact on placental efficiency/compromise. Considering that it is not safe or ethical to collect placental or fetal tissue samples during early and mid-pregnancy, only a few studies have shown EDC effects on placentae and fetal tissue using aborted or early-terminated pregnancies (610). Advancements in noninvasive approaches such as Doppler imaging and fetal ultrasound may aid in understanding adverse impacts of EDCs during pregnancy. Ongoing efforts to model the maternal-fetal interface with advancements in culture conditions and development of the trophoblast organoid and placenta-on-a-chip model (611) may also pave the way for more rapid assessments of EDC effects, and gain mechanistic insights.

Need for Consideration of Paternal Contribution

Much of this review has focused on the maternal exposure to EDCs and their impact on gestational and birth outcomes. Although half of each fetus’ nuclear chromosomal complement comes from the father, paternal contributions toward birth outcomes are rarely considered. Limited studies have looked at associations in which either paternal occupational exposure to (612) or urinary measures of (613, 614), EDCs such as flame retardants, metals, BPA, and phthalates, are associated with poor gestational outcomes such as implantation failure and hypospadias. These findings highlight the need to consider paternal exposure in assessing the impact of EDCs before birth.

Need to Establish Early Biomarkers

Another approach to overcome the limitations of early detection is to identify biomarkers of maternal, placental, and fetal dysfunction induced by EDCs especially those with short half-lives (593). Advanced “-omics” technologies, including genomic, epigenomic, transcriptomic, proteomic, metabolomics, microbiomics and metagenomics, can be used to identify novel unbiased markers (615) to predict maternal, placental, and fetal changes attributable to EDC effects and aid in early detection, thus providing opportunities for early intervention to eliminate detrimental effects of EDCs. Such approaches are being used for clinical conditions, including placental changes associated with PE (616, 617).

Need for Assessment of Endocrine-Disrupting Chemical Measures Following Safe Interventions

Lifestyle factors, including dietary choices, not only dictate exposure, but can also interact with EDCs to affect their outcome as discussed earlier. Because pregnancy is a major susceptibility window for developmental effects of the EDC, reproductive health professionals have started counseling on lifestyle changes (618). Changes in use of personal care products, diet, and physical activity are reported during pregnancy (619, 620) that may affect EDC exposure and action. Assessment of EDC measures and outcomes are therefore needed before and following counseling during the preconception and prenatal periods to encourage lifestyle changes that reduce EDC exposure, such as eating a well-balanced diet, avoiding processed food, reducing the use of cosmetics, and avoiding the use of pesticides (621).

Need for Linking Association with Causality

In human, risk assessments are made by studying associations between exposure domain or the presence of EDC in the maternal or fetal milieu with the outcome being studied. These studies merely point to potential relationships and do not address cause and effect (622), which can only be surmised through randomized trials. Some examples of such studies are the release of BPA from resin-based dental restorations and its effects on executive functioning in children in New England Children’s Amalgam Trial (623) and a randomized double-blind, placebo-controlled study on long-term treatment with phytoestrogens on endometrial hyperplasia (624). For ethical and safety reasons, studies involving the administration of EDCs during human pregnancy outcomes are not possible. Therefore, observational studies in humans need to be paired with experimental studies involving animal models that manifest similar developmental trajectories as humans to establish causal relationships (625). Studies with animal models, which allow the direct administration of EDCs in a controlled manner, can help establish cause and effect across subsequent generations. These studies needs to be carried out in translationally relevant animal models, because mechanistic studies in animal models that are developmentally dissimilar to humans may not aid in translation to humans. Use of humanized rodent models may help to a certain degree to overcome this challenge (626). Another aspect to consider is that these studies should model human internal exposure levels of EDCs derived from epidemiological studies. For instance, administration of a mixture of epidemiologically determined concentrations of various phthalate metabolites present in pregnant women in Illinois to pregnant mice showed sex-specific adverse effects such as reduction in anogenital distance in females, but not males, and long-term effects on reproductive function in both sexes (627, 628).

Need for Implementing Better Statistical Modeling Approaches

The assessment of EDC effects during pregnancy presents many challenges, including dissimilarity in the chemical structure and distribution, latency of effects, as well as confounders provided by differences in age, BMI, ethnicity, diet, geographical location, and occupation. A large body of data is being gathered through observational epidemiologic studies that incorporate medical records, EDC measures at different gestational time points, “-omic” measures (transcriptome, metabolome, epigenome, inflammasome, etc.), outcome assessments in children at different ages that are highly variable, complex, and poorly structured. These add to the levels of complexity that are inadequately addressed by traditional toxicity tests to predict population‐level effects (629). Additionally, obtaining insights through knowledge gained via complex, high-dimensional and heterogeneous data sets poses a major challenge in terms of understanding the true impact of EDCs. Mathematical modeling tools are therefore required to provide insight into relationships between EDC exposure and adverse outcomes. Tools such as mediation analysis are available to help determine the contribution of an intermediate that links the exposure-outcome association (630), and these techniques have undergone considerable improvement over recent decades to handle multiple mediators and interactions (631). With advancements in data science, computing, and machine learning, the application of Artificial Intelligence (AI) is another means through which machine learning algorithms can be applied to analyze large volumes of complex data (big data) like EDC mixtures and confounding variables to identify biomarkers and link to disease outcomes (632). By deciphering patterns, making predictions, pointing to new effective paradigms to obtain end-to-end learning models and developing a holistic interpretable blueprint to facilitate applicability, AI can help in risk assessment. As these determinations from AI are believed often to exceed the accuracy and efficiency of people (633) the mounting challenges due to the complicated nature of the data and insufficient domain knowledge can be overcome. The use of these sophisticated data science and AI approaches accounting for all confounding variables arising from multiple chemicals, periods of exposure, dose, sex and demographic diversity in determining EDC pregnancy outcomes is the way of the future.

In conclusion, evidence to date points to EDCs directly and indirectly affecting the maternal, placental, and fetal milieu and engaging many intermediaries that can be targeted for intervention and exposure mitigation. However, establishment of cause and effect relationships relative to pregnancy perturbations and outcomes stemming from EDC burden would require careful consideration and integration of several aspects identified earlier. These data should inform regulatory assessments of chemicals that are introduced into the environment. Currently, the EPA’s Endocrine Disruptor Screening Program screens chemicals for potential effects on estrogen, androgen, and thyroid hormone systems, although a multitude of evidence points to the disruption of metabolic pathways (634). Therefore, legislative efforts requiring testing a broader range endocrine disruption is needed to prevent introduction of new chemicals and regulate the use of existing chemicals. Therefore, a concerted effort by clinicians, epidemiologists, basic scientists, and statisticians would be required to meet these needs and aid in the development of informed legislation for mitigation of exposure to existing and newer EDCs, reduce the burden of short-lived EDCs and develop interventions to overcome the effects of persistent EDCs. These and the continued education of healthcare providers and the general public will aid in preventing adverse outcomes and promoting a healthy mother and child both in the short- and long-term.

Glossary

Abbreviations

AHR

arylhydrocarbon receptor

AI

artificial intelligence

AR

androgen receptor

As

arsenic

BMI

body mass index

BPA

bisphenol A

BPB

bisphenol B

BPS

bisphenol S

CHAMACOS

Center for the Health Assessment of Mothers and Children of Salinas

Cd

cadmium

Ce

cesium

CHD

chlordane

Cr

chromium

CRP

C-reactive protein

DAP

dialkylphosphate

DBP

dibutyl-phthalate

DDE

diphenyl dichloro ethylene

DDT

dichlorodiphenyltrichloroethane

DEHP

phthalate di(2-ethylhexyl)-phthalate

DES

diethylstilbestrol

DMP

dimethyl-phthalate

DOHaD

developmental origin of health and disease

E2

estradiol

EDCs

endocrine-disrupting chemicals

EPA

US Environmental Protection Agency

ESR

estrogen receptor

GDM

gestational diabetes mellitus

GR

glucocorticoid receptor

GxE

gene-environment interaction

HCB

hexachlorobenzene

Hg

mercury

HMW

high molecular-weight

IFNG

interferon γ

IGF1

insulin-like growth factor

IL

interleukin

IUGR

intrauterine growth restriction

LGA

large for gestational age

LINE

long interspersed nuclear elements

LMW

low-molecular-weight

lncRNA

long noncoding RNA

MBzP

mono-benzyl phthalate

MCINP

mono(carboxy-isononyl) phthalate

MCPP

mono- (3-carboxypropyl) phthalate

MCOMHP

mono (6-COOH-2-methylheptyl) phthalate

miRNA

microRNA

Mn

manganese

MRA

mixture risk assessment

mRNA

messenger RNA

MEHHP

Mono(2-ethyl-5-hydroxyhexyl) phthalate

MEOHP

mono-(2-ethyl-5-oxohexyl) phthalate

Ni

nickel

NIH

National Institutes of Health

NHANES

National Health and Nutrition Examination Survey

NOAEL

no observed adverse effect level

PAHs

polyaromatic hydrocarbons

Pb

lead

PBDE

polybrominated diphenyl ether

PCB

polychlorinated biphenyl

PE

preeclampsia

PFAS

perfluorinated alkylated substance

PFOA

perfluorooctanoic acid

PFOS

perfluorooctane sulfonic acid

PGR

progesterone receptor

PM

particulate matter

PPAR

peroxisome proliferator-activated receptor

PTB

preterm birth

RXR

retinoid X receptor

SGA

small for gestational age

T

testosterone

T3

3,5,3′-triiodothyronine

T4

thyroxine

TBT

tributyltin

TM

trimester

TNF

tumor necrosis factor

TSH

thyrotropin

V

vanadium

VEGF

vascular endothelial growth factor

Zn

zinc

Additional Information

Disclosures: The authors have nothing to disclose.