Periconceptional Use of Phthalate-Containing Medications and Secondary Sex Ratio

Introduction

Secondary sex ratio (SSR) is defined as the ratio of number of males to females at birth and is considered a meaningful end point for indicating adverse effects of environmental exposures (Organisation for Economic Co-operation and Development 2018). The SSR in humans has been relatively stable over time in showing a slight male excess in births (51.4%) (James 2008). However, in recent years, some industrialized nations have witnessed a decline in SSR (Chao et al. 2019). Reasons for variability in the SSR are unclear. A prevailing hypothesis is that exposure to endocrine-disrupting chemicals in animals and humans influences SSR via changes in periconceptional hormonal concentrations (James 1987). Nevertheless, human studies of periconceptional exposure to endocrine-disrupting compounds and SSR have produced inconsistent results (Terrell et al. 2011).

Phthalates are endocrine-disrupting chemicals found in selected consumer products and medications. Humans exposed to phthalate-containing medication classes have up to 50-fold higher urinary phthalate-metabolite concentrations (Hernández-Díaz et al. 2009). In the Longitudinal Investigation of Fertility and the Environment (LIFE), a preconception cohort study of 220 singleton live births, higher maternal urinary concentrations of mono-isobutyl phthalate [risk ratio $\left(\text{RR}\right)=1.28$, 95% confidence interval (CI): 1.06, 1.54], mono-benzyl phthalate ($\text{RR}=1.31$, 95% CI: 1.08, 1.58), and mono-n-butyl phthalate ($\text{RR}=1.24$, 95% CI: 1.01, 1.51) were associated with a male excess in live births (Bae et al. 2015). In LIFE, exposure reflected total phthalate exposure from all sources [e.g., household items (vinyl flooring), personal care products, household cleaners, food, and medications] but was most likely due to nonmedication sources. To our knowledge, no studies have directly examined phthalate exposure from medication use and SSR. Phthalates in Danish pharmaceuticals include ortho-phthalates [diethyl phthalate (DEP), the parent compound of mono-ethyl phthalate; and dibutyl phthalate (DBP), the parent compound of mono-isobutyl phthalate and mono-n-butyl phthalate), and phthalate polymers [hydroxypropyl methylcellulose phthalate (HPMCP), cellulose acetate phthalate (CAP), and polyvinyl acetate phthalate (PVAP)] (EMA 2014).

We evaluated the association between periconceptional use of phthalate-containing medications and SSR among Danish women.

Methods

Using personal identifiers assigned to all Danish residents, we linked data from several Danish health registries (Laugesen et al. 2021). Data on medication excipients in pharmaceuticals marketed during the period 2004–2017 from the Danish Medicines Agency were linked with individual-level prescription data from the Prescription Register. We used the Anatomical Therapeutic Chemical Classification System (ATC) and unique product codes to identify phthalate content of prescriptions (Ennis et al. 2018), classified as ortho-phthalates or phthalate polymers. We defined preconceptional exposure as prescription redemption in the 3 months before conception (14 d of gestation) and early pregnancy exposure as prescription redemption during the first trimester (conception until 90 d of gestation).

We obtained data on offspring sex and potential confounders from the Medical Birth Register and National Patient Register, including maternal age, calendar year at delivery, parity, first-trimester smoking, and first-trimester body mass index (BMI). The in vitro fertilization (IVF) register contained data on use of assisted reproductive technologies (ART) and prescriptions filled for fertility medications in the month before the index pregnancy, including gonadotropins and other ovulation-inducing drugs (ATC codes: G03G, H01CA, H01CC, L02AE, N04BC).

During the period 2004–2018, we identified 894,547 live-born singleton births. We excluded pregnancies without known gestational duration (8,714; 1%), women not residing in Denmark continuously for at least 1 y before pregnancy until delivery (13,456; 2%), women with preconceptional exposure before 2004 (59,012; 7%) or first trimester exposure ending after 2017 (28,510; 3%), and women without prescriptions redeemed during either exposure window (332,521; 42%), leaving 452,334 singletons born to 339,876 mothers. To minimize confounding by indication, we further restricted analyses to women exposed to medications available in both phthalate-containing and phthalate-free versions (78,165 singletons born to 51,397 mothers). Thus, we compared SSR among women taking phthalate-containing medication relative to women who took the same medication that did not contain phthalates. Clinicians and pharmacists were considered blinded as to phthalate content of the prescribed medications.

We estimated odds ratios (OR) and 95% CI using generalized estimating equations to account for multiple births per mother. We selected confounders based on the literature and causal diagrams. These included maternal age at birth ($<25$, 25–29, 30–34, 35–39, $\ge 40\text{\hspace{0.17em}years of age}$), calendar year of infant’s birth (2004–2008, 2009–2013, 2014–2017), parity (1, 2, $\ge 3\text{births}$), and history of infertility prior to current pregnancy (ever vs. never). We performed secondary analyses further adjusted for paternal phthalate-containing medication use during preconception and analyses restricted to first births during the study period (regardless of parity) or births conceived without use of ART or fertility medications. We assigned missing values for smoking and BMI to the modal value for each variable. We used Stata software (version 16.1; Stata Corp.) for statistical analyses.

Results

Exposed and unexposed women were comparable with respect to age, BMI, parity, smoking status, and concomitant medication use. Exposed women were less likely to conceive in the later years of the study, to have an infertility history, or to have used ART to conceive their index pregnancy (Table 1).

Table 1.

Baseline characteristics of Danish women with and without exposure to phthalate-containing medications during preconception or early pregnancy (2004–2017).

	Exposed			Unexposed
	Total exposeda	Ortho-phthalate exposed	Phthalate-polymer exposed	Phthalate unexposed (exposed to study medication)b	Unexposed to study medication but exposed to other medicationsc
All (No. of pregnancies)	4,653	3,765	888	46,057	401,624
Age at birth [mean year (SD)]	25.3 (11.2)	24.7 (11.5)	27.8 (9.5)	25.3 (11.1)	26.0 (10.5)
$<25$	681 (15%)	605 (16%)	76 (9%)	6,584 (14%)	49,438 (12%)
25–29	1,385 (30%)	1,147 (30%)	238 (27%)	14,689 (32%)	124,643 (31%)
30–34	1,633 (35%)	1,309 (35%)	324 (36%)	15,282 (33%)	142,258 (35%)
35–39	794 (17%)	590 (16%)	204 (23%)	7,794 (17%)	70,016 (17%)
$\ge 40$	160 (3%)	114 (3%)	46 (5%)	1,708 (4%)	15,269 (4%)
Year of birth
2004–2008	2,905 (62%)	2,682 (71%)	223 (25%)	15,547 (34%)	122,374 (30%)
2009–2013	1,012 (22%)	728 (19%)	284 (32%)	16,147 (35%)	147,830 (37%)
2014–2017	736 (16%)	355 (9%)	381 (43%)	14,363 (31%)	131,420 (33%)
Body mass index (${\mathrm{kg}/\mathrm{m}}^2$)
$<20$	467 (11%)	386 (11%)	81 (10%)	3,982 (9%)	40,926 (11%)
20–24	2,074 (47%)	1,686 (48%)	388 (49%)	19,346 (44%)	194,255 (50%)
25–29)	1,087 (24%)	845 (24%)	242 (31%)	11,538 (26%)	94,296 (24%)
$\ge 30$	770 (18%)	617 (17%)	153 (20%)	9,396 (21%)	57,391 (15%)
Missing	255	231	24	1,795	14,756
Parity, births
1	2,068 (44%)	1,683 (45%)	385 (43%)	20,189 (44%)	187,094 (47%)
2	1,672 (36%)	1,339 (36%)	333 (38%)	15,821 (34%)	144,716 (36%)
$\ge 3$	913 (20%)	743 (20%)	170 (19%)	10,047 (22%)	69,814 (17%)
Smoking status
Nonsmoker	3,626 (80%)	2,896 (79%)	730 (84%)	36,078 (80%)	340,044 (87%)
Smoker	894 (20%)	759 (21%)	135 (16%)	8,681 (20%)	50,425 (13%)
Missing	133	110	23	1,298	11,155
Infertility historyd	243 (5%)	138 (4%)	105 (12%)	3,436 (7%)	53,431 (13%)
Use of ART to conceive index pregnancy	148 (3%)	83 (2%)	65 (7%)	2,129 (5%)	40,532 (10%)
Use of fertility medications to conceive index pregnancy	276 (6%)	199 (5%)	77 (9%)	2,692 (6%)	51,433 (13%)
Concomitant study-related medicationse [mean (SD)]	1.1 (0.4)	1.1 (0.4)	1.2 (0.4)	1.0 (0.2)	(All unexposed)

Note: Expressed as numbers and column percents or means and SD. ART, assisted reproductive technology; IVF, in vitro fertilization; SD, standard deviation.

^aExposed in preconception or first trimester of pregnancy. Exposure group for all subsequent analyses.

^bExposed study medication in a phthalate-free version.

^cExcluded from analysis. Unexposed to study medications but exposed to other prescription medications (Ennis et al. 2018).

^dReceived medical treatment for infertility (ART) in a previous pregnancy attempt, as recorded in IVF Register.

^eExposed to other medications available in both phthalate-containing or phthalate-free versions (Ennis et al. 2018).

The probability of male birth was similar among exposed (51.0%) and unexposed (51.1%) women (Table 2). Among exposed women, the probability of male birth was 52.1% for preconception exposure to ortho-phthalates, 52.6% for first-trimester exposure to ortho-phthalates, 48.1% for preconception exposure to polymers, and 50.2% for first-trimester exposure to polymers. Relative to no exposure, adjusted ORs for male birth were 1.04 (95% CI: 0.93, 1.17) for preconception ortho-phthalate exposure, 1.06 (95% CI: 0.88, 1.28) for first-trimester ortho-phthalate exposure, 0.89 (95% CI: 0.76, 1.03) for preconception polymer exposure, and 0.97 (95% CI: 0.81, 1.17) for first-trimester polymer exposure. Results were similar among first births and births conceived without use of ART or fertility medications. Paternal use of phthalate-containing medications during preconception also showed little association with SSR: The probabilities of male birth for paternal preconception use of ortho-phthalates and polymers were 51.1% and 51.4%, respectively, and adjusted ORs for male preconception exposure to ortho-phthalates and polymers vs. no exposure were 0.97 (95% CI: 0.88, 1.06) and 0.98 (95% CI: 0.84, 1.15), respectively. Further adjustment for paternal use of ortho-phthalates (4.2%) and polymers (1.4%) had minimal influence on the maternal results (Table 2).

Table 2.

Preconception or early pregnancy use of phthalate-containing medications and secondary sex ratio among Danish women (2004–2017).

	Number of offspring		Percent male (%)	Unadjusted OR (95% CI)	Adjusted OR (95% CI)a	Adjusted OR (95% CI)b
	Male	Female
All births
Unexposed	23,963	22,982	51.0	1.00c	1.00c	1.00c
Exposedd
Preconception or first trimester	2,379	2,274	51.1	1.00 (0.94, 1.06)	1.00 (0.94, 1.07)	1.00 (0.94, 1.07)
Ortho-phthalates only	612	562	52.1	1.04 (0.93, 1.17)	1.04 (0.93, 1.17)	1.04 (0.93, 1.17)
Polymers only	441	447	49.7	0.95 (0.83, 1.08)	0.95 (0.83, 1.08)	0.95 (0.83, 1.08)
Ortho-phthalates and polymers	1,326	1,265	51.2	1.01 (0.93, 1.09)	1.01 (0.93, 1.09)	1.01 (0.93, 1.09)
Preconception	1,644	1,618	50.4	0.97 (0.91, 1.04)	0.97 (0.91, 1.05)	0.97 (0.91, 1.05)
Ortho-phthalates only	507	467	52.0	1.04 (0.92, 1.18)	1.04 (0.91, 1.18)	1.05 (0.92, 1.19)
Polymers only	341	368	48.1	0.89 (0.76, 1.03)	0.89 (0.76, 1.03)	0.89 (0.76, 1.03)
Ortho-phthalates and polymers	789	780	50.3	0.97 (0.88, 1.07)	0.97 (0.88, 1.07)	0.97 (0.88, 1.07)
First trimester	1,037	961	51.9	1.03 (0.95, 1.13)	1.03 (0.94, 1.13)	1.03 (0.94, 1.13)
Ortho-phthalates only	231	208	52.6	1.07 (0.88, 1.28)	1.06 (0.88, 1.28)	1.06 (0.88, 1.28)
Polymers only	233	231	50.2	0.97 (0.81, 1.16)	0.97 (0.81, 1.17)	0.97 (0.81, 1.16)
Ortho-phthalates and polymers	572	519	52.4	1.06 (0.94, 1.19)	1.06 (0.93, 1.19)	1.06 (0.93, 1.19)
First births
Unexposed	17,701	16,984	51.0	1.00c	1.00c	1.00c
Exposedd
Preconception or first trimester	2,000	1,886	51.5	1.02 (0.95, 1.09)	1.02 (0.95, 1.09)	1.02 (0.95, 1.09)
Ortho-phthalates only	570	523	52.1	1.05 (0.93, 1.18)	1.04 (0.92, 1.18)	1.04 (0.92, 1.18)
Polymers only	301	298	50.3	0.97 (0.82, 1.14)	0.97 (0.83, 1.14)	0.97 (0.82, 1.14)
Ortho-phthalates and polymers	1,129	1,065	51.5	1.02 (0.93, 1.11)	1.02 (0.93, 1.11)	1.02 (0.93, 1.11)
Non-ART births e
Unexposed	22,891	21,860	51.1	1.00c	1.00c	1.00c
Exposedd
Preconception or first trimester	2,303	2,202	51.1	1.00 (0.94, 1.06)	1.00 (0.94, 1.06)	1.00 (0.94, 1.06)
Ortho-phthalates only	606	557	52.2	1.04 (0.93, 1.17)	1.04 (0.92, 1.17)	1.04 (0.92, 1.17)
Polymers only	412	411	50.1	0.96 (0.83, 1.10)	0.96 (0.83, 1.10)	0.96 (0.83, 1.10)
Ortho-phthalates and polymers	1,285	1,234	51.1	0.99 (0.92, 1.08)	0.99 (0.92, 1.08)	0.99 (0.92, 1.08)

Note: ART, assisted reproductive technology; CI, confidence interval; OR, odds ratio.

^aAdjusts for maternal age at birth, calendar year of infant’s birth, parity, and history of infertility prior to current pregnancy.

^bAdjusts for all covariates in footnote a plus paternal use of phthalate-containing medications during preconception.

^cReference group for all column comparisons.

^dPreconception or first-trimester exposure to medications containing ortho-phthalates or phthalate polymers.

^eExcludes births conceived with use of ART or fertility medications.

Conclusion

In this Danish registry-based cohort study, maternal use of phthalate-containing medications during preconception or early pregnancy was not appreciably associated with SSR.