Child health: is it really ART that we need to be concerned about?

Abstract

Concerns remain about the health of children conceived by infertility treatment. Studies to date have predominantly not identified substantial long-term health effects after accounting for plurality, which is reassuring given the increasing numbers of children conceived by infertility treatment worldwide. However, as technological advances in treatment arise, ongoing studies remain critical for monitoring health effects. To study whether the techniques used in infertility treatment cause health differences, however, remains challenging due to identification of an appropriate comparison group, heterogeneous treatment, and confounding by the underlying causes of infertility. In fact, the factors that are associated with underlying infertility, including parental obesity and other specific male and female factors may be important independent factors to consider. As such, the following review will summarize key methodological considerations in studying children conceived by infertility treatment including the evidence of associations between underlying infertility factors and child health.

INTRODUCTION

The health of children conceived with assisted reproductive technologies (ART) has always been of concern to couples considering infertility treatments and for health practitioners. Worldwide, the number of births conceived by ART continues to climb.^1–3 As of 2015, ART contributed to 1.7% of all births in the United States (US).² Almost all ART procedures performed in the US are in vitro fertilization (IVF), of which 69% included intracytoplasmic sperm injection (ICSI).² Though other types of fertility treatment, including ovulation induction (OI) and intrauterine insemination (IUI), are commonly used as the first line of treatment, less data has been gathered to track their trends over time. Estimates of births conceived with non-ART treatments range from 3–7%.⁴

There have been numerous reviews published on the topic of child health after conception by infertility treatment, including one published last year in the current journal⁵ and our own focused on cardio-metabolic health.⁶ Hence, the current review is not intended to exhaustively review the existing literature in this area. However, in brief, evidence suggests that babies conceived by ART are born earlier and smaller than their peers, which lead to concerns for neurodevelopmental and other long-term health outcomes given the evidence from developmental origins of health and disease research. Nevertheless, in general we^7,8 and others have found no major differences in development or growth in early childhood after accounting for plurality. Longer term studies are needed, as well as studies with a substantial group of children conceived by non-ART treatments. Less investigated are early signs of developmental differences such as fetal growth and placental development.⁹ Specific techniques and treatment effects such as embryo culture media and oocyte vitrification also continue to be evaluated.

The goal of the present review is to discuss the epidemiologic hurdles to studying the health impact of conception by infertility treatment, including how perhaps the reasons for infertility may drive independent health effects long-term. Under the developmental origins of health and disease paradigm, specific conditions, such as maternal obesity, may lead to long-term offspring health risks ranging from obesity to neurodevelopmental delay. These conditions are also related to the risk of infertility. As such, we will also review common risk factors for male and female infertility and discuss potential impacts on developmental programming in order to address how best to investigate whether ART itself possesses risks to child health.

EPIDEMIOLOGIC RESEARCH INTO HEALTH OF CHILDREN CONCEIVED BY INFERTILITY TREATMENT

There are some unique methodological considerations for studying children conceived by infertility treatment. Table 1 summarizes the considerations below which are discussed in greater detail.

Table 1.

Key Methodological Considerations in Studying Children Conceived by Infertility Treatment

Challenges with studies on ART	Primary concerns
Selection of appropriate populations	Using children conceived spontaneously as a comparison poses challenges in discerning whether differences are due to the treatment procedures or the underlying infertility Socio-economic differences in couples who undergo infertility treatment and the general population may limit comparability, and future studies should seek to evaluate differences in socio-economically and racially diverse populations
Confounding	Studies may be confounded by a number of factors, and perhaps most importantly underlying infertility Other factors to consider include: thyroid function, BMI, age, race, SES, nutrition, environment, mental health, health of both parents
Study design	Lack of RCTs Lack of long follow-up in cohort studies
Representativeness	Comparison between studies is often challenging due to protocol heterogeneity in treating infertility (ovarian stimulation, ICSI, ART) and evolving technologies and techniques Technological advances in techniques make any findings today not necessarily relevant for future children conceived by more advanced techniques

Population Considerations

Couples undergoing infertility treatment comprise a unique study population, making it challenging to choose the correct comparison group in epidemiologic studies. Many studies investigating health effects of children conceived by infertility treatment have utilized a group of children conceived spontaneously as a comparison. An inherent problem with using a comparison group of children conceived spontaneously is that it becomes difficult to discern whether the adverse health outcomes potentially observed in the offspring are a result of the infertility treatment, or rather the underlying infertility that indicated treatment. If the study question is to examine the effect of infertility treatment on the risk of health outcomes, comparisons should be made between children spontaneously conceived and children conceived through treatment, either both derived from fertile couples or both from infertile couples. The former strategy may be unethical and the latter may be untenable. Hence, other strategies are necessary.

One such strategy is to compare health of children conceived with ART against children conceived by subfertile couples in the same clinic who did not go on to use ART as a method. The Millennium Birth Cohort, for example, examined children who were conceived by subfertile couples. They did not find any differences between the ART (n=101) and the subfertile (n=485) groups with regards to childhood behavioral difficulties at 5 and 7 years of age.¹⁰ In a separate investigation using the same cohort, the researchers found that subfertility was associated with risk of asthma in children along with the ART group.¹¹ Some difficulties in using this strategy include not having equal sample sizes of children followed, and residual confounding, as couples who undergo ART may likely have more severe infertility than subfertile couples which remain a source of unclear bias. For rare outcomes, a combination of large registry data may be necessary. One such large retrospective cohort in Massachusetts found that use of ART and diagnosed subfertility were both associated with higher prevalence of birth defects compared to spontaneous conceptions.¹² Their definition of subfertility may more represent that of resolved infertility, as it included a past indication of infertility treatment but not in the current cycle.¹² Such a definition may provide a closer counterfactual comparison to remove underlying infertility rather than no conception after 12 months of trying, but may also have the limitation of misclassification for any current fertility treatment use missed from documentation or a change in partner. Nevertheless, it is an important strategy to keep in mind for trying to disentangle underlying infertility from findings on treatment effects.

Apart from underlying infertility, population comparability is also challenged by the socioeconomic gap between couples who can afford infertility treatment for conception and couples in the general population. According to data from 2014, the average cost of one fresh ART cycle in the US has been estimated as $13,548 US dollars, which may be a substantial proportion of annual household income in the general population.¹³ Until recently, ART was not part of mandated health insurance coverage, and as a result, those seeking treatment for infertility were of higher socioeconomic background, limiting the generalizability of findings from the epidemiologic studies to the general population. Since the 1980’s, 14 states in the US have passed laws mandating the insurance coverage of infertility treatments. As insurance coverage for these treatments has become more widely available for a broader population, there is increasing heterogeneity in the socioeconomic background of children born to ART treatments. Despite increased availability of treatment, evidence suggests that racial and economic disparities persist even in mandated states, perhaps due to lack of access and awareness of treatments.¹⁴ As such, efforts should be made in future epidemiologic studies to evaluate long-term outcomes children conceived by infertility treatment among socioeconomically and racially heterogeneous populations, and also consider whether the study population is paying for treatment out-of-pocket.

Heterogeneous treatments

Adding to the difficulties of separating specific treatment effects from confounding by underlying infertility is that the selection of treatment options is a revolving process. Treatment selection primarily depends on the factors related to infertility diagnosis and the patients’ response to specific types of treatment. When treatment fails, a more aggressive protocols may be used. Grouping all ART together creates a heterogeneous exposure group. Studies are now beginning to separate the types of hormonal treatment used for ovarian stimulation to better investigate specific long-term health effects. One such study using large registry data found that progesterone treatment rather than other hormonal exposures, increased autism risk.¹⁵

Other studies have also evaluated the effects of ICSI as it is becoming increasingly common for the treatment of male factor infertility. In fact, its use in the US has increased from 36.4% in 1996 to 76.2% in 2018, with the largest increase among cycles without male factor infertility.¹⁶ The safety of ICSI has been questioned due to reports suggesting an increase in congenital anomalies in children who were conceived with ICSI.^17–19 The majority of studies utilize registry data to capture these rare outcomes.^16,18–21 As infertility may be a risk factor for birth defects, studies comparing ICSI to the general population do not properly address the risk of the ICSI procedure itself.^19,22–24 For example, the indication for ICSI is an important factor, as male factor infertility has been associated with an elevated risk of congenital malformations. A cohort of 1,586 patients undergoing prenatal invasive screening after ICSI found that abnormal fetal karyotypes were increased in fetuses conceived after ICSI in comparison to the general population (1.4 versus 0.3 to 0.4%). Of the inherited anomalies experienced in the ICSI population, 17 of 22 were due to a structural anomaly in the father. In addition, de novo mutations were increased for semen concentration less than 20 × 10⁶/ml.²⁵ The higher rate of genitourinary defects in male infants conceived with ICSI implicates that male factor infertility is likely an independent contributing factor.¹⁸

Evolving technologies

Ongoing surveillance and research are also required to keep up with the technological advances being made in infertility treatment. In 2013, the American Society for Reproductive Medicine (ASRM) announced that oocyte cryopreservation was no longer considered experimental.²⁶ Vitrification is the technique of cryopreservation that solidifies the cell and extracellular environment into a glass-like state without the formation of ice.²⁷ Due to this recent change, no childhood studies have been conducted but short-term perinatal outcomes have been examined. A prospective case control study compared pregnancy outcomes of previously vitrified oocytes from egg donors to fresh autologous oocytes.²⁸ There was no significant difference between the gestational age at delivery or birth weight.²⁸ However, this study potentially selected on the highest achievable outcomes, as the donor population is consistently young and healthy. More studies are needed to assess clinical outcomes in the general population. An observational cohort study found that embryo quality was not impacted by use of cryopreserved oocytes. However, morphokinetic studies did show that embryo development was delayed after use of cryopreserved oocytes. It is unknown if this may translate to pregnancy or neonatal outcomes.²⁹

Another recent change in practice is that transfer of day 5 embryos has been advocated by ASRM to decrease the risk of multiple gestations and allow selection of the highest quality embryo.³⁰ The development of the embryo to the blastocyst stage provides an additional competence measure, as the activation of the embryonic genome occurs at the 8-cell stage.³¹ Blastocysts transfers are reported to result in higher live birth rates in comparison to cleavage stage embryos in good prognosis patients.³² It is suggested that increased implantation rates are a result of improved endometrial synchronization.^32,33 However, good prognosis patients are more likely to be selected for a day 5 transfer as they are more likely to have a greater number of embryos available for transfer. A randomized study including women who were less than 37 years of age had a higher pregnancy rate after embryo transfer on day 5 compared to day 3 (35.6% vs 23.7%).³⁴ A retrospective study found that in patients less than age 40, live birth rates per initiated cycle were significantly higher after transfer of single blastocyst versus cleavage-stage embryo (37.8% vs 31.3%).³⁵ A retrospective clinical study found that with increasing age, patients were less likely to develop an embryo to the blastocyst stage. However, this was likely due to the decrease number of oocytes retrieved. Patients, regardless of age, achieved similar pregnancy rates per blastocyst transferred.³⁶

Extended culture of embryos to day 5 may be associated with adverse pregnancy outcomes. A total of 12,712 singleton births recorded in the Canadian ART Register database were reviewed to identify the impact on obstetric outcomes. Day 5 embryo transfer was associated with an increased risk of preterm birth in day 5 versus day 3 embryo transfer (odds ratio [OR] 1.32, 95% confidence interval [CI] 1.17–1.49).³⁷ A retrospective review of Society for Assisted Reproductive Technology (SART) 2004–2006 data found an increased risk of preterm delivery in singleton pregnancies after day 5 transfer compared with day 3 transfer (18.6% vs 14.4 %; adjusted OR 1.39, 95% CI 1.29–1.50).³⁸ These findings were also consistent for donor recipient patients and patients who were considered to have a good prognosis. A meta-analysis also reported that day 5 embryo transfers were associated with an increase in congenital anomalies in comparison to cleavage stage embryos (OR 1.29, 95% CI 1.03–1.62).³⁹ A retrospective registry study reviewing 4,819 singleton births after blastocyst transfers, 25,747 cleavage-stage transfers and nearly 2 million spontaneous conceptions found no increase in birth defects. However, perinatal mortality and risk of placental complications were higher in blastocyst transfers.⁴⁰ Blastocyst transfer may impact child development as well. A randomized control study found that the type of culture media was associated with birth weight and growth during the first 2 years of life.⁴¹ Culture media has been shown to influence expression of genes critical for embryo development in animal studies.⁴² Future studies are necessary to determine the impact of blastocyst transfer on the development of the fetus and throughout the lifetime of the individual. Such changes in techniques also drive the necessity for ongoing surveillance.

EPIDEMIOLOGIC CUNUNDRUM: ART VS. UNDERLYING INFERTILITY

Apart from population and technological considerations, underlying infertility has been a source of confounding giving rise to questions regarding whether associations observed with child health are truly due to treatment effects. Parental health has an impact on child outcomes beginning at delivery.⁴³ Despite a wealth of research into developmental programming from maternal pregnancy conditions, fewer studies have focused on pre-conceptional health and how infertility risk factors of both partners may play a role. Part of the difficulties lie in measuring participants prior to pregnancy as well as accuracy of participant reported diagnoses of some of the infertility disorders. Below summarizes the evidence for factors which children’s health has been investigated, including parental age, parental obesity, dietary intake, thyroid function, depression, and other female gynecologic conditions.

Parental age

Advanced maternal age (≥35 years) increases risks of pregnancy and neonatal complications.^44–46 It can also increase risks of chromosomal abnormalities. Based on increasing rates of preterm delivery and other adverse neonatal outcomes, one anticipates higher risks of adverse long-term outcomes. However, risks of long-term impact on childhood growth and development from advanced maternal age may be modified by postnatal circumstances. Women of older age at childbirth tend to have better socioeconomic status and it is difficult to tease apart those benefits conferred. Indeed, in a study of children from five birth cohorts in low to middle income countries, stunting at 2 years of age (OR 064; 95% CI 0·54–0·77) and failure to complete secondary schooling (OR 0·59; 95% CI 0·48–0·71) were both protected by advanced maternal age as compared to children of mothers of 20–25 years of age.⁴⁷ A retrospective cohort study tried to tackle the question of longer term outcomes by using hospital data linking 200,000 deliveries in Israel and their outcomes through 18 years of age.⁴⁸ They did not identify clear differences in risk of childhood morbidities (i.e., central nervous system/brain, lymphoma, leukemia) related to maternal age at 35–39 (hazard ratio [HR] 1.06; 95% CI 0.76–1.48) or even at 40–50 years (HR 0.73; 95% CI: 0.36–1.46) when compared to 20–34 years.⁴⁸ Yet, other studies have observed high risks of childhood cancers including retinoblastoma and leukemia with increasing maternal age.⁴⁹ Further complicating the impact of maternal age in ART is the use of donor or autologous oocytes and the unclear comparison group in those cases. Live-birth rates decline with increasing maternal age in ART but this is particularly true for autologous cycles rather than for donor cycles.⁵⁰ Hence, the maternal age effects are suspected to be due to decreasing oocyte quality over time, but aside from short-term outcomes, the impact of maternal age over the life-course may be counterbalanced by positive socioeconomic influences with older age, particularly in terms of childhood growth and development although perhaps less so for risks of cancer.

Paternal age may also play a role that sometimes is difficult to isolate from the effects of maternal age as couples tend to have correlated age. For example, the Childhood Leukemia International Consortium identified increased risks of acute lymphoblastic leukemia associated with advanced paternal age with mixed results for advanced maternal age, owing to the collinearity of the age information.⁵¹ Accumulating evidence suggests, however, that paternal age and indeed paternal risk factors in general, may be important to assess. A very comprehensive systematic review of this evidence was published on paternal factors.⁵² The authors located the most articles for investigations into paternal age, with much fewer for other aspects (i.e., obesity, smoking) due to fewer studies with paternal information. For short-term outcomes, they found “little or no difference” in rates of preterm, low birth weight or small-for-gestational age due to advanced paternal age. A very slight increase in stillbirths (with low evidence) and in birth defects (with moderate evidence) was also observed. For longer term outcomes, paternal age has been related to risks for autism⁵³ but evidence of other psychiatric disorders is lacking. Little evidence suggested increased risks for childhood cancer, obesity, or diabetes.⁵²

Parental Obesity

About a third of all US adults are obese (BMI≥30), making it a critical concern. Male and female obesity increases risk of infertility. For females, obesity is associated with anovulation, miscarriage and longer time to pregnancy. The impact of adiposity is primarily hypothesized to be through hormonal disruption as adipose tissue is endocrinologically active. For males, increases in DNA fragmentation and epigenetic effects have been observed and are beginning to be further elucidated. Few studies have investigated childhood outcomes related to paternal BMI.⁵² In the short-term, paternal height rather than BMI is associated with birthweight. In terms of long-term effects, the Upstate KIDS Study observed that paternal obesity (BMI≥30) was associated with increased odds of failing the personal-social domain of the Ages and Stages Questionnaire (ASQ), rated by parents at 4, 8, 12, 18, 24, 30, and 36 months of age.⁵⁴ Although this association was not strong, it was independent of maternal BMI. In fact, maternal obesity was associated with failing the fine motor domain. Severe parental obesity (BMI≥35 in both parents) was additionally associated with failing the problem-solving domain. Findings from Suren and colleagues using data from MoBa, a large birth cohort from Norway, has also demonstrated a stronger, linear association between paternal BMI rather than maternal BMI with risk of developing autism.⁵⁵

As the role of epigenetics have come to be demonstrated, however, empirical evidence suggests that the paternal contribution goes beyond traditional heridity.^56,57 In 2015, scientists from INSERM demonstrated in a clever experiment that sperm/testis RNA from male mice fed a western diet injected into once-cell embryos of control parents fed standard diets, lead to progeny with higher weight and altered glucose tolerance.⁵⁸ They further identified two microRNAs (i.e., miR-19b and miR-29a) to be particularly dysregulated in male mice fed western diets. Specifically, miR-19b micro-injection increased offspring body weight. Apart from miRNA, other small piwi RNA (piRNA) or transfer RNA (tRNA) are also suspected to play a role.⁵⁶

Many more studies have investigated the role of maternal obesity on short- and long-term outcomes in child health. Pre-pregnancy BMI is also associated with pregnancy complications such as gestational diabetes and preeclampsia which in turn lead to neonatal complications such as preterm delivery and suboptimal fetal growth (i.e., macrosomia or growth restriction). Apart from the observed associations in Upstate KIDS mentioned above, other studies have also identified maternal obesity as a risk factor for a range of neurodevelopmental outcomes.⁵⁹

Dietary factors: Vitamin D

Moreover, obesity is tied to energy balance, dietary intake, and micronutrient metabolism. For instance, obese individuals are more likely to have low circulating concentrations of 25-hydroxy vitamin D [25(OH)D] since the lipid soluble vitamin is sequestered by adipose tissue. Vitamin D’s potential link with fertility is suggested by evidence that its receptor is prevalent on both male and female reproductive tissues in rodent models.⁶⁰ In one study, female rats deficient in vitamin D experienced a 75% reduction in fertility.⁶¹ Investigations in humans have largely focused on pregnancy outcomes of women undergoing ART. As recently reviewed by Pacis et. al., vitamin D deficient women [25(OH)D<20ng/mL] have a lower percent implantation and clinical pregnancy per embryo transfer compared with vitamin D sufficient [25(OH)D≥30ng/mL] women.⁶²

Maternal vitamin D levels, in turn, have been investigated with regards to child health. Gestational and cord blood 25(OH)D concentrations have been associated with neurodevelopmental outcomes, such as mental or psychomotor development, among infants and toddlers in most,^63–68 but not all,⁶⁹ studies. In early childhood, low 25(OH)D concentrations from gestational or newborn samples have been related to externalizing symptoms,⁷⁰ attention deficit hyperactivity disorder (ADHD)-like symptoms^70–72 and autism diagnosis.^73–75 Interestingly, no association between gestational or cord blood 25(OH)D and lifetime ADHD diagnosis has been detected.^76,77 In middle childhood and early adolescence, gestational 25(OH)D has been inversely associated with language ability⁷⁸ and verbal fluency.⁷⁹ However, it is unclear if the associations detected between 25(OH)D and markers of infant development translate into other cognitive or behavioral problems later in life since, with one exception,⁶⁹, no relation between low 25(OH)D concentrations and measures of cognitive ability (i.e., IQ or scholastic achievement)^68,70,77,80 or behavior problems^{68,69,77,78,80,81} in middle childhood or adolescence has been found. Nutritional studies remain difficult to conduct. Unlike vitamin D, many other factors such as protein intake or even caloric consumption, are not well captured by biomarkers. Self-report is also limited. Future epidemiologic studies of maternal nutrition may leverage on new technologies such as metabolomics.

Polycystic Ovarian Syndrome (PCOS)

PCOS is a complex endocrine disorder affecting 5–16% of women.^82–84 It is the most common identifiable cause of infertility, and is characterized by menstrual irregularities, hyperandrogenism, polycystic ovaries, and frequently, obesity and insulin resistance.⁸² PCOS is a strongly heritable disorder, with studies suggesting that over 70% of PCOS is linked to genetic causes.^85,86 Mothers with PCOS are at higher risk of numerous pregnancy complications, including gestational diabetes, pregnancy-induced hypertension, preeclampsia, pre-term birth, and infant mortality, which may all influence offspring health.^87,88 Women with PCOS may be hyperandrogenic during pregnancy, and endocrine function during pregnancy may impact fetal development. While these changes do not appear to impact infant growth patterns, accumulating evidence suggests that in utero exposure to a hyperandrogenic environment is associated with neurodevelopmental disorders in offspring.^89–94 In the Upstate KIDS Study, maternal PCOS was associated with higher risks of developmental delays in several domains, particularly among girls.⁸⁹ This finding has been supported by several large studies in Europe using administrative data, which have found associations between maternal PCOS and/or elevated fetal testosterone and higher risks of autism, ADHD and pervasive developmental disabilities.^{90–92,95–99}

As with studies of potential health effects of IVF on offspring, studies of PCOS face several methodological challenges. PCOS is a heterogeneous condition with diagnostic criteria that have changed over time, and identification of true cases may be challenging. As PCOS is associated with higher risks of depression and mental health disorders, there has been some concerns that maternal mental health may confound the relationship between PCOS and offspring neurodevelopment, despite a lack of evidence that maternal mental health conditions cause PCOS. A large recent study in the United Kingdom addressed this concern by comparing offspring of women with PCOS to offspring of mothers without PCOS with similar mental health history, finding that PCOS was associated with higher risk ADHD and autism regardless of maternal mental health history.⁹² A second concern is that PCOS may be more frequently diagnosed in women pursuing fertility treatment, which in the US is expensive and may reflect a population with higher socioeconomic status (SES). In these cases, lower SES women with PCOS may be underrepresented in some studies if study selection is conditional on pursuit of fertility treatment. This could lead to greater misclassification of PCOS status by SES among a control or non-PCOS reference population compared to a case or PCOS-positive group in research studies. Due to potential associations of SES with PCOS and neurodevelopment, statistical adjustment for SES is an important consideration. However, despite concerns about misclassification of PCOS by SES, the consistency of estimates across different countries and health systems where diagnosis of PCOS may be less related to SES – such as European countries with national healthcare systems – suggests that associations between PCOS and neurodevelopmental disorders are less likely to be wholly attributable to SES or country-specific patterns of diagnosis. The hypothesis that maternal PCOS may increase risk for offspring neurodevelopmement is also supported by animal models, which indicate that a PCOS-like phenotype in mothers can have a host of metabolic and developmental implications for offspring.¹⁰⁰ Studies that show a relationship between fertility treatment and offspring health, particularly neurodevelopment, should consider the role of PCOS and other causes of infertility.

Thyroid function

Hypothyroidism affects 2–4% of reproductive-age individuals and is most often caused by thyroid autoimmunity.¹⁰¹ Thyroid hormones including thyroid stimulating hormone (TSH), thyroxine (T4) and triiodothyronine (T3) exert direct and indirect effects on normal ovarian function and may thus be relevant for female infertility. For example, granulosa cells contain T3 binding sites and serum levels of T3 and T4 have been shown to correlate with follicular fluid levels of these hormones.¹⁰² Thyroid hormones may also act indirectly on the hypothalamic-pituitary-ovarian axis via suppression of sex hormone-binding globulin (SHBG) activity and elevation of thyrotropin-releasing hormone (TRH).¹⁰³ Specifically, decreased binding activity of SHBG may increase free testosterone and estradiol, resulting in elevated androstenedione and estrone.

It is currently unclear whether the effects of thyroid hormones on sex steroid hormone levels and menstrual cycle length are sufficient to influence female fertility, as epidemiologic studies evaluating hypothyroid and infertility are scarce and conflicting. A cross-sectional study of women ages 18–39 reported significantly higher TSH levels among women with unexplained infertility as compared to controls (1.95 versus 1.66 mIU/L; P = 0.0003).¹⁰⁴ Moreover, twice as many women with unexplained infertility had TSH levels ≥ 2.5 mIU/L as compared to controls (P<0.05). These findings are consistent with those of Poppe et al, who observed significantly higher mean TSH levels among 197 infertile women presenting at a fertility clinic as compared to 100 parous women (1.7 +/− 8.8 versus 1.2 +/−0.7 mIU/L; P=0.05).¹⁰⁵ The association was particularly strong for women experiencing ovulatory disorder infertility as compared to control women (1.9 +/− 4.9 versus 1.2 +/−0.7 mIU/L; P=0.003). The presence of thyroid peroxidase antibodies (TPO-Abs) was also associated with 2.5 times the odds (95% CI 1.02–5.12) of infertility as compared to no TPO-Abs. In a more recent prospective study of euthyroid women, preconception subclinical hypothyroidism, as defined by TSH ≥2.5 mIU/L, was not associated with time to pregnancy, pregnancy loss, or live birth among healthy fecund women.¹⁰⁶ The presence of TPO-Abs antibodies was also not associated with any of these outcomes in that population.

Hypothyroidism seems to be related to disruption of ovarian function and menstrual cycle disturbance, which may subsequently lead to subfertility and indication for infertility treatment. Collectively, the existing evidence suggests that that overt hypothyroidism is positively associated with infertility, whereas subclinical hypothyroidism is not, though there is much debate about the relevant cut-point for differentiating the two.¹⁰⁷ Furthermore, it is largely unknown what the joint effects of hypothyroidism, infertility and ART treatment may have on the health outcomes of the offspring. Future studies should carefully consider these joint effects in well-designed prospective cohort studies.

Thyroid hormones are essential for growth and development of all organs during the fetal period and later. In the brain, they act through nuclear receptors and regulate gene expression influencing neurogenesis, neuronal migration, neuronal and glial cell differentiation, myelination, and synaptogenesis.^108–111 During the first weeks of gestation when the fetal thyroid gland is not fully functional, inadequate thyroid hormones in the mother can also be detrimental because the fetus solely relies on maternal transfer of hormones for growth and development.¹¹² Even though substantial physiological alteration occurs in thyroid function of pregnant women to maintain the need of the mother and fetus, thyroid dysfunction is a common clinical problem during pregnancy.¹¹³ If it remains undetected or inadequately treated, maternal overt hypothyroidism results in cognitive impairments in the offspring, even in the absence of neonatal hypothyroidism.¹¹⁴ Neuroimaging studies of children born to hypothyroid mothers have shown that brain morphological abnormalities may persist as the child ages despite maternal treatment.^115,116 In view of this evidence, guidelines recommend treatment of overt hypothyroidism during pregnancy to prevent adverse outcomes in both mother and child.^117,118

In recent years, emerging evidence from epidemiological studies suggests that milder forms of thyroid hormone insufficiency during gestation are also associated with suboptimal cognitive and behavioral outcomes in children. Subclinical hypothyroidism which is defined as high levels of TSH with T4 within the gestational age-specific normal range is associated with a reduction in children’s IQ.¹¹⁴ Differences in mental development and neuropsychological functioning have been reported in children born to women with subclinical hypothyroidism compared to euthyroid women.^119,120

Children born to women with isolated hypothyroxinemia (T4 values below the 2.5^th or 5^th percentile with TSH within the normal range) are at increased risk of suboptimal cognitive function¹²¹, language delay¹²², ADHD symptoms and other behavioral problems,^123–126 autistic symptoms,^127,128 motor function and executive function,¹²⁴ and schizophrenia.¹²⁹ One neuroimaging study reported an association of maternal hypothyroxinemia with smaller grey matter and cortex volumes and larger cerebellar white matter volumes in children at age 6 years.¹³⁰ Null associations have been reported as well.¹³¹

Grounded on convincing evidence from observational studies,^{121–125,127–130} two randomized clinical trials have examined the benefit of treatment on cognitive function in the children of women who underwent thyroid screening in early pregnancy and had mild thyroid insufficiency.^132,133 However, the trials have failed to demonstrate expected benefits of treatment and improvement in children’s IQ. Nonetheless, some speculate that the null findings are attributed to late interventions (after week 12 of gestation) or potential overtreatment.^134,135 Considering the lack of strong evidence on the association between milder forms of hypothyroidism and infertility, it is unlikely that subclinical hypothyroidism or hypothyroxinemia explain the potential impact of ART on the offspring’s neurodevelopment. Overt hypothyroidism in women, however, should be considered in studies of ART and child neurodevelopment.

Parental depression

Empirical evidence suggests that there is an association between depression and infertility.¹³⁶ For instance, a cross-sectional study in Hungary found that infertile women had significantly higher depression scores than their fertile counterparts (14.94 vs. 8.95, p<0.0001).¹³⁷ Similarly, a Polish study found higher mean depression scores associated with infertility among women (8.5 vs. 5.2, p<0.0001).¹³⁸ Men, although less studied, are not exempt to depressive symptoms associated with infertility.¹³⁹ Methodological limitations, including the cross-sectional design, small sample size, lack of appropriate comparison groups, and inadequate adjustment for confounding (particularly with history of depression) present a challenge in determining whether depression is a predictor or a consequence of infertility.¹⁴⁰

Experiencing depressive mood over time may have a negative influence on the regulation of reproductive hormones in male and female, or it may give rise to conditions such as obesity which in turn affects fertility.¹⁴¹ However, it is less clear whether infertility can be the underlying factor for depressive mood versus a cause of infertility. Given limitations of the literature, more methodologically rigorous studies are needed for a thorough understanding of the etiological direction of the association between depression and infertility. In particular, examining the timing of depression onset, duration, and history during the preconception period may improve the understanding of this relationship. Information on the timing and nature of depression may enable reproductive health researchers to establish a timeline for development of depression with respect to fertility status for couples.

Irrespective of the mode of conception, parental psychological health may negatively impact child outcomes. Perinatal depression (depression during pregnancy and postpartum period) particularly has a negative influence on infant and child development.^142,143 Various studies have shown that children born to women experiencing perinatal depression are at a higher risk for experiencing adverse behavioral and developmental outcomes including preterm birth¹⁴⁴, externalizing and internalizing behaviors^145,146, poor growth¹⁴⁷, and reduced cognitive functioning.¹⁴⁸ Fathers are also at risk for experiencing perinatal depression¹⁴⁹, though research on fathers’ depression status during this time period is scarce^150,151 and its effect on child outcomes is unclear. Several studies suggest fathers’ perinatal depression is associated with children’s adverse emotional status^152,153 however a more recent study found that this relationship is mediated by maternal depression and couple conflicts.¹⁵⁴

Depression during the perinatal period may be an indication of a more chronic condition with an onset in the preconception period. However, studies examining the onset, frequency, and duration of depression during preconception period are limited. Furthermore, despite accumulating evidence on the presence of perinatal depression in fathers^155,151, the literature on this topic including predictors of this condition —specifically the timing and the history of depression— and its association with infertility and child outcomes is very limited and remain important to assess.

Other female infertility factors

Aside from the aforementioned conditions indicated for infertility treatment, little is known about the long-term health outcomes of children conceived by ART to women with other conditions associated with infertility, such as endometriosis and fibroids. Endometriosis is associated with infertility, and accounts for up to 40% of all infertility causes among couples seeking treatment.¹⁵⁶ Importantly, endometriosis is associated with an inflammatory milieu.¹⁵⁷ Women with endometriosis undergoing ART experience a reduced rate of fertilization,¹⁵⁶ thought to be caused by excessive local production of pro-inflammatory cytokines including interleukins and tumor necrosis factor-alpha (TNF-α). In line with the inflammation hypothesis, women with endometriosis are at increased risk of hypertension and cardiovascular disease, indicating long-term health consequences associated with inflammation beyond just the reproductive years.^158,159 Uterine fibroids occur in up to 70% of women and are also associated with infertility.¹⁶⁰ Some have hypothesized that inflammation may also play a role in the pathogenesis of uterine fibroids, and women with fibroids appear to experience higher cardiovascular risk.¹⁶¹ Because inflammation is associated with adverse pregnancy and child health outcomes, future epidemiologic studies should evaluate long-term child health outcomes among those conceived with ART by mothers with uterine fibroids and endometriosis.

Note on Impact of Confounding

Despite the long list of possible related conditions reviewed above, a note must be made that the bias from confounding is dependent on its strength of association with the outcome of interest as well. It is usually best to draw a causal diagram¹⁶² depicting the model one wants to estimate associations for and determine which are true confounders and which may be mediators of associations (e.g., preterm delivery) that need not be adjusted for, or may require more specific types of analyses. Figure 1 shows an example of a conceptual diagram for examining the association between ART and the outcome of neurodevelopment. One can adjust for BMI, underlying infertility and socio-demographics (i.e., age, race, education) to adequately control for the confounders listed. In this case, even if underlying infertility does not directly impact neurodevelopment in some way, due to its association with upstream factors, its adjustment would remain a possible option for controlling for confounding of these other factors (e.g., thyroid function) that might not be well captured in a specific study but are known to lead to neurodevelopmental issues.

Figure 1.

Conceptual Diagram for Examining the Association between Use of Assisted Reproductive Technologies (ART) and Offspring Neurodevelopment.

However, even if there is no information collected on some of the indicators of infertility treatment in a specific study, the impact of confounding by indication for each specific research question can be estimated using bias analyses.¹⁶³ Such analyses may then help evaluate whether the missing confounding information would or would not have impacted the associations found in the study. Following the recommended reporting guidelines such as STROBE for observational research¹⁶⁴, unadjusted estimates should also be published along with models with successive adjustment models to better evaluate the impact of covariate adjustment. In addition, causal evidence for the confounders are required. For the above factors, many of which have been understudied, the true estimates of associations along with biological underpinnings between them and child health remain unclear. Further research is necessary to provide better estimates for bias analyses.

Conclusion

In sum, the evidence accumulated on the health of children after conception by ART (and less so for non-ART treatments) suggest that they are largely similar in development and growth as their peers. However, given the heterogeneity and continuing advancements of ART procedures, ongoing research and surveillance that tackles the existing methodological challenges is essential. Even if fertility treatment techniques themselves may not cause marked differences in long-term health, attention should also be given to understanding the impact of the causes of infertility including both male and female factors.