Maternal pre-pregnancy overweight/obesity, in-utero exposure to toxic heavy metals, and offspring age at peak height velocity: A prospective birth cohort study

Abstract

Introduction:

Research on in-utero heavy metal exposure shows inconsistent results on offspring pubertal timing. We aim to contribute to understanding the association between heavy metal exposures and age at peak height velocity (APHV), an indicator of pubertal timing. A second aim was to investigate the joint associations of heavy metal concentrations and previously known risk factors.

Methods:

We included 956 mother-child dyads from the Boston Birth Cohort. Heavy metal concentrations were measured in maternal red blood cells collected 24–72 h after delivery. Offspring APHV was calculated using mixed-effects growth curve models based on repeated height measurements over time. Firstly, we investigated maternal lead, cadmium and mercury level in association with APHV using multivariate linear regression models. Secondly, we estimated the joint associations of maternal pre-pregnancy overweight/obesity (or race/ethnicity) and heavy metal concentration with APHV, after adjusting for confounders. Thirdly, we applied Bayesian kernel machine regression (BKMR) model to investigate association between the heavy metal mixture and APHV.

Results:

Overall, prenatal heavy metal exposure was modestly inversely (but not statistically significantly) associated with earlier offspring APHV. Both childhood and maternal pre-pregnancy overweight/obesity were associated with an earlier APHV. Non-Hispanic Black males had an earlier APHV than other race/ethnicity males. Moreover, lead and cadmium exposure and maternal pre-pregnancy overweight/obesity were jointly associated with APHV in males only and females only, respectively. The joint exposure of maternal pre-pregnancy overweight/obesity and high lead concentration was associated with earlier APHV among males, and likewise maternal pre-pregnancy overweight/obesity and high cadmium concentration with earlier APHV among females. The joint associations of race/ethnicity and high concentrations of lead and mercury with APHV were observed in males.

Conclusions:

Our findings suggest that maternal pre-pregnancy overweight/obesity prevention and environmental toxic chemicals control may help attenuate the secular trend of early puberty, particularly among non-Hispanic Black individuals.

Graphical Abstract

Introduction

Early pubertal timing is associated with higher risk of depression (Conley and Rudolph, 2009; Sequeira et al., 2017), especially when children reach puberty significantly earlier or out of sync with their peers (Sequeira et al., 2017). Additionally, longer exposure to gonadotropin and growth-related hormones increase the risk of breast (Goldberg et al., 2020) and ovarian cancers (Yang et al., 2019).

Pubertal timing has accelerated over the past two decades (Farello et al., 2019; Walvoord, 2010). One explanation is that body mass index (BMI) has increased over the last four decades (Chen et al., 2021; Farello et al., 2019; National Academies of Sciences et al., 2019). Obesity induces metabolic mechanisms (Karaolis-Danckert et al., 2009) such as insulin resistance and reduces the level of sex hormone binding globulin (SHBG) which regulates sex hormone bioavailability (Dunger et al., 2006; Kalme et al., 2003). In turn, obese children may experience increased sex hormone levels (Dunger et al., 2006; Nakai et al., 1978).

Another pathway is through exposure to environmental toxicity, such as endocrine-disrupting chemicals (EDCs) or heavy metal exposure (Farello et al., 2019; Parent et al., 2003). In-utero toxic exposure has revealed associations between EDCs and childhood overweight/obesity (Huang et al., 2022), leading to earlier APHV (Chen et al., 2021). Among environmental metals, mercury (Hg) has been suggested to lead to earlier pubertal timing (Tan et al., 2009), while the associations of cadmium (Cd) (Bobrisheva-Pushkina et al., 2023; Interdonato et al., 2015; Johnson et al., 2003; Samuel et al., 2011; Stoica et al., 2000) and lead (Pb) on pubertal timing have produced controversial findings (Dearth et al., 2002; Doumouchtsis et al., 2009). Pb and Hg pass easily through the placenta from mother to child, while Cd cannot easily cross the placental barrier (Chen et al., 2014). Research specifically focusing on in-utero metal exposures and its association with menarche and precocious puberty have yielded contradictory findings (Jansen et al., 2018; Malin Igra et al., 2023; Wang et al., 2021). While some studies show that in-utero exposures to Pb, Cd and Hg are associated with earlier menarche and an increased risk of earlier pubertal timing (Ashrap et al., 2020; Malin Igra et al., 2023; Wang et al., 2021), other research suggests that in-utero Pb exposure is linked to later menarche (Jansen et al., 2018).

Most literature on pubertal timing has focused solely on girls (Belsky et al., 1991; Wierson et al., 1993), likely because it is easier to determine pubertal timing among girls using age of menarche (Belsky et al., 1991). However, pubertal acceleration is also seen among boys measured by APHV and the age at onset of the pubertal growth spurt (Aksglaede et al., 2008; Karaolis-Danckert et al., 2009). To date, studies on in-utero exposure to environmental metals have only been conducted among girls (Jansen et al., 2018; Malin Igra et al., 2023; Wang et al., 2021).

Genetics have been pointed out as a crucial determinant of menarche and Tanner staging milestones (Gluckman and Hanson, 2006; Mancini et al., 2022). Additionally, sociodemographic (Walvoord, 2010), perinatal (Gluckman and Hanson, 2006; Yang et al., 2023) and postnatal (Koziel and Jankowska, 2002; Walvoord, 2010) factors also play a role in earlier Tanner staging milestones in both males and females and menarche in females (Mancini et al., 2022; Walvoord, 2010). Important sociodemographic factors include race/ethnicity (Freedman et al., 2002; Kaplowitz et al., 2001) and socioeconomic status (SES) such as household income and parental education and occupation (Tremblay and Frigon, 2005). Literature focusing solely on females suggests that non-Hispanic Black females are more likely to experience earlier menarche compared to non-Hispanic White females (Freedman et al., 2002; Kaplowitz et al., 2001). However, data on males are limited. To fill this gap, we conducted a prospective birth cohort study to estimate whether in-utero exposure to heavy metals and maternal pre-pregnancy overweight/obesity along with race/ethnicity were individually or jointly associated with APHV, an important indicator of pubertal timing among both sexes in a predominantly non-Hispanic Black U.S. population.

Methods

Study population

This study was conducted using data from the Boston Birth Cohort (BBC), a prospective birth cohort study initiated at the Boston Medical Center (BMC) in Boston, MA, USA in 1998, as described previously (Pearson et al., 2022). The BBC was a predominantly non-Hispanic Black, low-income, and urban population, who are known to face longstanding and profound health disparities in the U.S. (Freedman et al., 2002; Kaplowitz et al., 2001; Pearson et al., 2022). Mother-infant pairs were enrolled 24-72 hours after delivery. At enrollment, each mother completed an interview administered questionnaire that assessed demographic factors and factors during pregnancy, and a blood draw. Since 2004, the BBC has followed 3416 mother-child pairs longitudinally, with a median follow-up length of 14.5 years. Of those, data on maternal red blood cell (RBC)-Pb, Cd, and Hg concentrations were available for 1,550 pairs. Among them, offspring APHV was available in 956 participants (470 females and 486 males), who were eligible for the subsequent analyses (Supplemental Figure 1). Demographic backgrounds of the 956 included participants and 2,458 excluded participants were compared (Supplemental Table 12). Among included participants, there were more non-Hispanic Black individuals, less smokers during pregnancy, less stressed mothers during pregnancy, less preterm birth, less low birthweight cases, and more childhood overweight/obesity compared to excluded participants.

Measurement of maternal RBC heavy metal levels

Pb, Cd, and Hg concentrations were measured in maternal RBCs collected 24-72 hours after delivery as a proxy for third trimester exposure levels. Measurement methods have been described previously (Chen et al., 2014). Lab analyses were performed by the Metals Laboratory, Environmental and Chemical Laboratory Services, New Jersey Department of Health, Trenton, NJ, USA. We used inductively coupled plasma mass spectrometry on an Agilent 8900 QQQ (Agilent Technologies Inc., Santa Clara, CA, USA), according to the Centers for Disease Control and Prevention quality control measures (Wang et al., 2021). The limit of detection (LOD) was 0.07 ug/dL for Pb, 0.100 ug/L for Cd, and 0.280 ug/L for Hg. Metal concentrations under the LOD were imputed with the LOD divided by the square root of two.

Identification of APHV

APHV was quantified using a growth curve analysis based on longitudinal height measurements, which were abstracted from medical records. The median (interquartile range) of height measurements per participant was 20 (14-29). Growth curve analyses for males and females were conducted separately, using SuperImposition by Translation And Rotation (SITAR), a well-validated, flexible model that uses subject-specific random effects to fit individual growth curves to a mean curve and provides unbiased estimates of APHV (Cole, 2018; Cole et al., 2014), using the R package “sitar”. First, we generated a set of growth curves with a mean growth curve as well as several random effects. Then, we used the random age intercept to address the subject-specific differences in the timing of the pubertal growth spurt. The Bayesian Information Criterion (BIC) was used to assess the best model fit.

Ascertainment of maternal pre-pregnancy overweight or obesity

Maternal pre-pregnancy height and weight data were extracted from a questionnaire administered 24-72 hours postpartum (Pearson et al., 2022). Maternal pre-pregnancy BMI was calculated as maternal pre-pregnancy body weight (kg) divided by the square of maternal height (m²). Maternal pre-pregnancy overweight or obesity was defined as BMI ≥ 25 kg/m² (Huang et al., 2022).

Definition of race/ethnicity

Information on race/ethnicity was retrieved from the maternal 24-72 hours postpartum questionnaire (Pearson et al., 2022). Self-identified race and ethnicity categories included African American, Cape Verdean, White, Hispanic, Asian, Haitian, Pacific Islander, mixed and other. In this study, we recoded African American, Cape Verdean and Haitian as non-Hispanic Black and White, Hispanic, Asian, Pacific Islander, mixed and other as Other.

Covariates

Sociodemographic factors included maternal age at delivery, maternal race/ethnicity and maternal education (Belsky et al., 2010; Freedman et al., 2002; Kaplowitz et al., 2001; Walvoord, 2010). Perinatal factors included maternal smoking status during pregnancy, maternal pre-pregnancy BMI and preterm birth (Voordouw et al., 2001; Yang et al., 2023). Missing data were observed in maternal education (6 participants, 0.63 %), maternal smoking during pregnancy (10 participants, 1.05 %), maternal pre-pregnancy overweight/obesity (47 participants, 4.92 %), and maternal stress (8 participants, 0.84 %). Mode imputation was conducted due to relatively low missing rates (Soom et al., 2022) and to maintain sample size for joint analyses. Complete case analysis (N = 879) was conducted as a supplementary analysis. Although income has been considered to be a potential confounder in other populations (Belsky et al., 1991; Jones et al., 2022), it was not included in the analyses due to a high rate of invalid responses 363 participants (37.97 %) answered ’don’t know’ and 101 (10.56 %) did not respond—resulting in a total of 464 invalid entries (48.5 %) (Che et al., 2023).

Childhood overweight/obesity was not included as a covariate because we considered it to be in the causative pathway. However, given its possible association with APHV (Chen et al., 2021), childhood overweight/obesity was added into the analysis as a crude check of mediation in supplementary analysis.

Statistical analyses

Statistical analyses were performed using the STATA SE statistical package, version 16.1 (StataCorp LP, College Station, ZTX, USA), R version 4.4.0 (R Core Team, 2024) and RStudio 2024.04.0 (Posit team, 2024). Sociodemographic factors stratified by sex were compared using t tests for continuous variables and Chi-squared tests for categorical variables. Maternal heavy metal concentration was initially log-transformed and dichotomized by the sex-specific median. Given that the APHV varies by sex, all analyses were conducted separately by sex. We used multivariate linear regression to examine the associations between continuous and dichotomized heavy metal exposures and APHV.

In the secondary analysis, interaction terms were examined. Accordingly, joint associations of maternal pre-pregnancy overweight/obesity and each metal concentration (Pb, Cd, Hg) were examined by multivariate linear regression. Similarly, the joint associations of race/ethnicity and each heavy metal concentration (Pb, Cd, Hg) were examined using multivariate linear regression. Additionally, we analyzed the joint association of heavy metal concentrations with childhood overweight/obesity, preterm birth, low birthweight, neighborhood environment and maternal stress. Neighborhood environment during pregnancy was defined by the National Neighborhood Equity Index (NNEI) in the maternal 24-72 hours postpartum questionnaire. Based on the 11 possible equity barriers, 0-2 was categorized as good, 3–11 as poor.

Given the correlation among concentrations of each heavy metal (Supplemental Table 1), we further conducted the mixture analysis to address potential interactions between exposures. We applied Bayesian kernel machine regression (BKMR) model (bkmr package for R) with a Markov Chain Monte Carlo (MCMC) algorithm of 50,000 iterations (Bobb et al., 2015). The cumulative associations between the three heavy metal concentrations and APHV sex-specific Z-score were investigated.

All the statistical analyses were stratified by sex. In the joint analyses, we used a common reference group to disentangle the individual and joint associations between maternal pre-pregnancy overweight/obesity (or race/ethnicity) and specific heavy metal exposures on APHV.

Ethical approval

The study protocol was approved by the Institutional Review Boards of BMC and the Johns Hopkins Bloomberg School of Public Health. Results were reported according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist (von Elm et al., 2007).

Results

956 mother-child pairs (470 females and 486 males) were included in this study (Supplemental Figure 1). 306 (65.11%) females were non-Hispanic Black, and 283 (58.23%) males were non-Hispanic Black. The mean (standard deviation (SD)) APHV was 11.01 (0.60) years among females and 12.85 (0.81) years among males. The medians (interquartile range (IQR)) of maternal Pb, Cd, Hg concentrations were 2.50 (1.72, 3.82) ug/dL, 0.71 (0.47, 1.09) ug/L, and 2.24 (1.11, 4.08) ug/L, respectively, among females and 2.36 (1.64, 3.80) ug/dL, 0.67 (0.44, 1.08) ug/L, and 1.91 (0.97, 3.44) ug/L, respectively, among males. Sociodemographic characteristics were similar between sexes, as shown in Table 1.

Table 1.

Characteristics of study population, stratified by sex.

Characteristics		Female (N=470)		Male (N=486)
		Number mean median	% SD IQR	Number mean median	% SD IQR	P value
Age at peak height velocity		11.01	0.60	12.85	0.81	<0.001
Heavy metal concentration a	Lead (ug/dL)	2.50	1.72, 3.82	2.36	1.64, 3.80	0.864
	Cadmium (ug/L)	0.71	0.47, 1.09	0.67	0.44, 1.08	0.783
	Mercury (ug/L)	2.24	1.11, 4.08	1.91	0.97, 3.44	0.100
Maternal pre-pregnancy BMI (kg/m2)		27.46	14.81	26.67	15.95	0.074
Maternal pre-pregnancy OWO	No	206	43.83	214	44.03	0.751
	Yes	264	56.17	272	55.97
Race b	Non-Hispanic Black	306	65.11	283	58.23	0.200
	Others	164	34.89	203	41.77
Maternal age at delivery (years)		28.78	6.50	29.13	6.65	0.416
Education attainment	High school and below	299	63.62	307	63.17	0.966
	College and above	171	36.38	179	36.83
Smoking status during pregnancy	Non-Smoker	398	84.68	404	83.13	0.552
	Smoker	72	15.32	82	16.87
Diabetes in pregnancy	Non-Diabetic	411	87.45	412	84.77	0.232
	Gestational and existing diabetes	59	12.55	74	15.23
Maternal perceived stress during pregnancy	Low	195	41.49	200	41.15	0.982
	Middle or High	275	58.51	286	58.85
Neighborhood environment c	Good	245	52.13	246	50.62	0.495
	Poor	225	47.87	240	49.38
Gestational age at delivery (weeks)		37.99	3.42	37.79	3.30	0.355
Birthweight (g)		2955.06	794.86	3016.98	830.44	0.240
Preterm birth	No	366	77.87	357	73.46	0.112
	Yes	104	22.13	129	26.54
Low birthweight	No	366	77.87	370	76.13	0.523
	Yes	104	22.13	116	23.87
Childhood OWO	No	264	56.17	271	55.76	0.900
	Yes	206	43.83	215	44.24
Marital status d	Non-married	312	66.38	324	66.67	0.009
	Married	158	33.62	162	33.33

^aHeavy metal concentrations were reported as medians (IQR) because data were skewed to the left.

^bAfrican American, Cape Verdean and Haitian were coded as non-Hispanic Black and White, Hispanic, Asian, Haitian, Pacific Islander, mixed and other as Others.

^cNeighborhood environment during pregnancy was defined by the National Neighborhood Equity Index (NNEI). Based on the 11 possible equity barriers, 0-2 was categorized as good, 3–11 as poor.

^dMarital status categories included married, widowed, divorced, separated and single. Choices except married was categorized as non-married.

Abbreviations: %: percentage; SD: standard deviation; IQR: interquartile range, BMI: body mass index, OWO: overweight/obesity.

Bolded results indicate P value<0.05.

956 participants were analyzed. Age at peak height velocity significantly differed among sex. Demographic backgrounds, maternal heavy metal exposure, maternal pre-pregnancy overweight/obesity, maternal race did not differ among sexes.

Maternal heavy metal concentrations and offspring pubertal timing

Table 2 shows the associations between individual heavy metal concentrations and APHV among both sexes after adjusting for maternal age, race/ethnicity, education, smoking during pregnancy, maternal pre-pregnancy BMI and preterm birth. No statistically significant associations were observed, though there was a trend of earlier APHV for all heavy metals (i.e. Pb, Cd, and Hg) among both sexes. Results were similar in complete case analysis shown in Supplemental Table 13.

Table 2.

Associations between individual heavy metal exposures and age at peak height velocity, stratified by sex.

		Female				Male
		N	β	95%CI	P value	N	β	95%CI	P value
Pb	Low	235	Ref			243	Ref
	High	235	−0.02	(−0.13, 0.10)	0.778	243	−0.04	(−0.19, 0.11)	0.602
Log2(Pb) a		470	−0.02	(−0.08, 0.05)	0.607	486	0.02	(−0.06, 0.10)	0.609
Cd	Low	235	Ref			243	Ref
	High	235	−0.06	(−0.17, 0.05)	0.298	243	0.03	(−0.12, 0.19)	0.653
Log2(Cd) a		470	−0.03	(−0.09, 0.04)	0.382	486	−0.002	(−0.08, 0.08)	0.955
Hg	Low	235	Ref			243	Ref
	High	235	−0.02	(−0.13, 0.09)	0.689	243	−0.02	(−0.16, 0.13)	0.812
Log2(Hg) a		470	−0.01	(−0.05, 0.03)	0.589	486	−0.001	(−0.05, 0.05)	0.974
Maternal OWO	No	206	Ref			214	Ref
	Yes	264	−0.14	(−0.25, −0.02)	0.018	272	−0.16	(−0.31, −0.02)	0.024
Childhood OWO	No	264	Ref			271	Ref
	Yes	206	−0.22	(−0.33, −0.12)	<0.001	215	−0.42	(−0.55, −0.28)	<0.001
Race b	Others	164	Ref			203	Ref
	NHB	306	−0.10	(−0.21, 0.02)	0.097	283	−0.27	(−0.41, −0.13)	<0.001

^alog-transformed.

^bAfrican American, Cape Verdean and Haitian were coded as non-Hispanic Black and White, Hispanic, Asian, Haitian, Pacific Islander, mixed and other as Others.

Abbreviations: β: coefficient; CI: confidence interval; Pb: lead, Cd: cadmium, Hg: mercury, OWO: overweight/obesity, BMI: body mass index, NHB: Non-Hispanic Black, APHV: age at peak height velocity.

Bolded results indicate P value<0.05.

Adjusted for maternal age, race/ethnicity, education, smoking during pregnancy, maternal pre-pregnancy BMI and preterm birth.

956 participants were analyzed. No statistically significant association were observed between maternal heavy metal exposure and offspring pubertal timing, though there was a trend of earlier APHV shown for all metals (i.e. Pb, Cd, and Hg) among both sexes.

Joint associations of maternal heavy metal concentrations and maternal pre-pregnancy overweight or obesity

Table 3 shows that among females, joint exposure to maternal pre-pregnancy overweight/obesity and high Cd concentration (coefficient −0.20, 95%CI: −0.37, −0.04) was associated with earlier APHV, compared to females whose mothers had no pre-pregnancy overweight/obesity and low Cd concentration. Among males, joint exposure to maternal pre-pregnancy overweight/obesity and high Pb concentration was associated with earlier APHV (coefficient −0.23, 95% CI: −0.52, −0.10), compared to males whose mothers had no pre-pregnancy overweight/obesity and low Pb concentration. For Hg exposure in females, joint exposure to maternal pre-pregnancy overweight/obesity and low Hg concentration was associated with earlier APHV compared to the reference group (no maternal pre-pregnancy overweight/obesity and low Hg concentration) (coefficient −0.23, 95% CI: −0.39, −0.08). However, joint exposure to maternal pre-pregnancy overweight/obesity and high Hg concentration showed a weaker negative association with child APHV (coefficient −0.17, 95% CI: −0.33, −0.02). For Hg exposure in males, joint exposure to both maternal pre-pregnancy overweight/obesity and high Hg concentration, no maternal pre-pregnancy overweight/obesity but high Hg concentration, maternal pre- pregnancy overweight/obesity but low Hg concentration was associated with earlier APHV, but the weakest in children with maternal pre-pregnancy overweight/obesity and high Hg concentration (coefficient −0.23, 95% CI: −0.44, −0.03) (Table 3). Results were similar in the complete case analysis shown in Supplemental Table 14.

Table 3.

Joint associations of heavy metal concentrations and maternal pre-pregnancy overweight/obesity with age at peak height velocity, stratified by sex.

		Female				Male
Maternal OWO	Metal	N	β	95%CI	P value	N	β	95%CI	P value
	Pb	470				486
No	Low	110	Ref			110	Ref
No	High	96	0.01	(−0.16, 0.18)	0.927	104	−0.02	(−0.24, 0.20)	0.834
Yes	Low	125	−0.12	(−0.28, 0.04)	0.130	133	−0.18	(−0.38, 0.03)	0.086
Yes	High	139	−0.16	(−0.32, 0.00)	0.056	139	−0.23	(−0.52, −0.10)	0.032
	Cd	470				486
No	Low	108	Ref			99	Ref
No	High	98	−0.02	(−0.19, 0.14)	0.782	115	0.09	(−0.13, 0.31)	0.409
Yes	Low	127	−0.11	(−0.27, 0.04)	0.158	144	−0.14	(−0.34, 0.07)	0.192
Yes	High	137	−0.20	(−0.37, −0.04)	0.016	128	−0.15	(−0.37, 0.07)	0.192
	Hg	470				486
No	Low	114	Ref			109	Ref
No	High	92	−0.13	(−0.29, 0.04)	0.125	105	−0.25	(−0.47, −0.04)	0.021
Yes	Low	121	−0.23	(−0.39, −0.08)	0.003	134	−0.40	(−0.60, −0.20)	<0.001
Yes	High	143	−0.17	(−0.33, −0.02)	0.029	138	−0.23	(−0.44, −0.03)	0.024

Abbreviations: β: coefficient; CI: confidence interval; Pb: lead, Cd: cadmium, Hg: mercury, OWO: overweight/obesity, BMI: body mass index, APHV: age at peak height velocity.

Bolded results indicate P value<0.05.

Adjusted for maternal age, race/ethnicity, education, smoking during pregnancy and preterm birth.

956 participants were analyzed. Joint exposure to maternal pre-pregnancy overweight/obesity and high Cd concentration led to earlier APHV among females, compared to females with no maternal pre-pregnancy overweight/obesity and low Cd concentration. Among males, joint exposure to maternal pre-pregnancy overweight/obesity and high Pb concentration was associated with earlier APHV, compared to males with no maternal pre-pregnancy overweight/obesity and low Pb concentration.

Joint associations of maternal heavy metal concentrations and maternal race/ethnicity

Table 4 shows the joint associations of race/ethnicity and heavy metal concentrations with APHV. Joint exposure to non-Hispanic Black race/ethnicity and high Pb concentration with earlier APHV (coefficient −0.23, 95% CI: −0.52, −0.10), compared to males whose mothers had no pre-pregnancy overweight/obesity and low Pb concentration. For Hg exposure in females, joint exposure to maternal pre-pregnancy overweight/obesity and low Hg concentration was associated with earlier APHV compared to the reference group (no maternal pre-pregnancy overweight/obesity and low Hg concentration) (coefficient −0.23, 95% CI: −0.39, −0.08). However, joint exposure to maternal pre-pregnancy overweight/obesity and high Hg concentration showed a weaker negative association with child APHV (coefficient −0.17, 95% CI: −0.33, −0.02). For Hg exposure in males, joint exposure to both maternal pre-pregnancy overweight/obesity and high Hg concentration, no maternal pre-pregnancy overweight/obesity but high Hg concentration, maternal pre-pregnancy overweight/obesity but low Hg concentration was associated with earlier APHV, but the weakest in children with maternal pre-pregnancy overweight/obesity and high Hg concentration (coefficient −0.23, 95% CI: −0.44, −0.03) (Table 3). Results were similar in the complete case analysis shown in Supplemental Table 14.

Table 4.

Joint associations of heavy metal concentrations and race/ethnicity with age at peak height velocity, stratified by sex.

Race/ethnicity a	Metal	Female				Male
		N	β	95%CI	P value	N	β	95%CI	P value
	Pb	470				486
Others	Low	104	Ref			128	Ref
Others	High	60	−0.09	(−0.28, 0.10)	0.340	75	0.13	(−0.09, 0.36)	0.243
Non-Hispanic Black	Low	131	−0.15	(−0.30, 0.01)	0.066	115	−0.13	(−0.33, 0.07)	0.216
Non-Hispanic Black	High	175	−0.12	(−0.27, 0.03)	0.106	168	−0.29	(−0.48, −0.11)	0.002
	Cd	470				486
Others	Low	99	Ref			132	Ref
Others	High	65	−0.14	(−0.33, 0.05)	0.148	71	0.05	(−0.18, 0.29)	0.647
Non-Hispanic Black	Low	136	−0.14	(−0.30, 0.01)	0.069	111	−0.27	(−0.47, −0.07)	0.009
Non-Hispanic Black	High	170	−0.16	(−0.32, −0.01)	0.036	172	−0.25	(−0.43, −0.06)	0.009
	Hg	470				486
Others	Low	91	Ref			110	Ref
Others	High	73	0.02	(−0.16, 0.21)	0.815	93	0.04	(−0.18, 0.26)	0.734
Non-Hispanic Black	Low	144	−0.06	(−0.22, 0.09)	0.424	133	−0.23	(−0.43, −0.03)	0.027
Non-Hispanic	High	162	−0.11	(−0.27, 0.04)	0.161	150	−0.28	(−0.48, −0.09)	0.005

^aAfrican American, Cape Verdean and Haitian were coded as non-Hispanic Black and White, Hispanic, Asian, Haitian, Pacific Islander, mixed and other as Others.

Abbreviations: β: coefficient; CI: confidence interval; Pb: lead, Cd: cadmium, Hg: mercury, BMI: body mass index, APHV: age at peak height velocity.

Bolded results indicate P value<0.05.

Adjusted for maternal age, education, smoking during pregnancy, maternal pre-pregnancy BMI and preterm birth.

956 participants were analyzed. Joint exposure to non-Hispanic Black race/ethnicity and high Pb concentration were associated with earlier APHV in males, compared to males with other race/ethnicity and low Pb concentration.

Joint associations of maternal heavy metal concentrations and maternal race/ethnicity

Table 4 shows the joint associations of race/ethnicity and heavy metal concentrations with APHV. Joint exposure to non-Hispanic Black race/ethnicity and high Pb concentration were associated with earlier APHV in males (coefficient −0.29, 95%CI: −0.48, −0.11). Among males, joint exposure to non-Hispanic Black race/ethnicity and high Hg concentration (coefficient −0.28, 95%CI: −0.48, −0.09), and joint exposure to non-Hispanic Black race/ethnicity but low Hg concentration (coefficient −0.23, 95%CI: −0.43, −0.03) both were associated with earlier APHV compared to the reference group (i.e., non-Hispanic Black race/ethnicity and low Hg concentration) (Table 4). Results were similar in complete case analysis shown in Supplemental Table 15.

Exploring potential interactions

As shown in Supplemental Table 2, in girls, there was no interaction between maternal overweight/obesity (or race/ethnicity) and maternal heavy metal concentrations on age at peak height velocity; notably, in boys, the interaction term was significant between maternal overweight/obesity and maternal Hg, and interaction term between race/ethnicity and maternal Pb. As complementary analyses, stratification by maternal pre-pregnancy overweight/obesity was additionally conducted (Supplemental Table 3). Stratification by race/ethnicity was additionally conducted (Supplemental Table 4).

Mixture analyses

Given the correlation among concentrations of each heavy metal (Supplemental Table 1), we further conducted the mixture analyses to address potential interactions between exposures. In the mixture analyses between heavy metal concentrations and APHV using the BKMR model (Figure 1), we found a null relationship in females and non-significant inverse trend of the metal mixture and APHV z-score in males.

Figure 1.

Bayesian kernel machine regression between the heavy metal mixture and age peak height velocity stratified by sex

Abbreviations: BMI: body mass index, APHV: age at peak height velocity, Pb: lead, Cd: cadmium, Hg: mercury.

Adjusted for maternal age, race/ethnicity, education, smoking during pregnancy, maternal pre-pregnancy BMI, preterm birth.

Males show a decrease in APHV, meaning earlier maturation, as heavy metal concentration increases. Females show similar trend but weaker.

Overall effect of heavy metal mixture (lead, cadmium, mercury) on the sex-specific age at peak height velocity z-score. Each point and line represent the estimate and 95%CI for exposure-effect association when all exposures (i.e., Pb, Cd, and Hg) are fixed at the corresponding percentiles; the figure shows the trend of association when all exposure percentiles simultaneously change from P10 to P90.

Joint associations of maternal heavy metal concentrations and other factors

Examination of joint associations of heavy metal concentrations and factors including childhood overweight/obesity, preterm birth, low birthweight, neighborhood environment and maternal stress during pregnancy in relation to APHV are presented in Supplemental Tables 5-7. Childhood overweight/obesity was significantly associated with earlier APHV for both sexes regardless of heavy metal concentrations. Children who experienced preterm birth or low birthweight and low heavy metal concentrations had later APHV compared to children who were not born preterm or low birthweight and were exposed to low heavy metal concentrations. Perinatal social factors such as neighborhood environment and maternal stress during pregnancy did not show clear associations with APHV.

Sensitivity analyses

After further adjusting for childhood overweight/obesity, the associations of maternal pre-pregnancy overweight/obesity and high heavy metal concentrations with APHV lost statistical significance (Supplemental Table 8). On the other hand, results between race/ethnicity and APHV remained similar (Supplemental Table 9). Overall, maternal pre-pregnancy overweight/obesity overall was not associated with heavy metal concentrations except for a negative association between Cd among males (OR 0.66, 95% CI: 0.44, 0.98) (Supplemental Table 10). Non-Hispanic Black race/ethnicity was associated with higher Pb concentration among both sexes (females OR 2.50, 95% CI: 1.65, 3.80, males OR 2.77, 95% CI: 1.85, 4.13) and higher Cd concentration among both sexes (females OR 1.90, 95% CI: 1.26, 2.87, males OR 2.92, 95% CI: 1.97, 4.34) (Supplemental Table 11).

Discussion

Our results suggest possible associations between in-utero Pb, Cd, Hg exposure and earlier APHV, though the associations did not reach statistical significance. The joint association of high metal concentration and maternal pre-pregnancy overweight/obesity led to earlier APHV, specifically for Cd exposure among females, and Pb exposure among males. Among males, we found a joint association of high Cd and Hg concentration and non-Hispanic Black race/ethnicity with earlier APHV. This is the first study to investigate the impact of prenatal exposure to Pb, Cd, Hg on the risk of early pubertal timing among both sexes in a high-risk U.S. population.

Although children with heavy metal exposures had earlier APHV, the association was not statistically significant. One possible reason for non-significance may be explained by the measurement methods. Firstly, in-utero heavy metal exposure was measured through maternal blood samples, not umbilical cord blood. Heavy metals such as Hg, Pb and Cd are known to pass through the placenta (Iyengar and Rapp, 2001; Zheng et al., 2014), though placenta penetration may differ by metal (Iyengar and Rapp, 2001; Zheng et al., 2014) (e.g., Pb is known to have high penetration (Iyengar and Rapp, 2001)). It is possible that the placenta traps some portion of the metal and prevents it from reaching the fetus, resulting in mismatch between heavy metal concentrations in maternal blood versus in the fetus (Caserta et al., 2013). However, previous studies comparing heavy metal levels in maternal blood during pregnancy and umbilical cord blood showed high correlations (Caserta et al., 2013).

In this study, pubertal timing was measured using APHV, a well-established measurement (Aksglaede et al., 2008; Sanders et al., 2017; Yoshii and Tanaka, 2018) that can be used in both sexes.

Maternal pre-pregnancy overweight/obesity and early offspring pubertal timing

In this study, maternal pre-pregnancy overweight/obesity was associated with earlier offspring APHV. Previous literature has reported an association between maternal pre-pregnancy overweight/obesity and childhood overweight/obesity (Si et al., 2023), as well as an association between childhood overweight/obesity and earlier APHV (Chen et al., 2021). Several mechanisms may explain the association between childhood overweight/obesity and earlier pubertal timing. Firstly, leptin - which is a hormone secreted from the adipose tissue - stimulates gonadotropin-releasing hormone which is upstream to the HPG axis (Martos-Moreno et al., 2010; Shalitin and Phillip, 2003), and may trigger earlier pubertal timing. Secondly, early-life weight gain leads to insulin resistance increasing insulin and adrenal androgen secretion (Casazza et al., 2008; Dunger t et al., 2006). This reduces sex hormone-binding globulin increasing free sex hormone levels (Casazza et al., 2008; Dunger et al., 2006). Thirdly, while further research on epigenetic pathways between nutritional status and pubertal timing are needed (Anderson et al., 2024), preliminary findings suggest that epigenetic alterations may be induced through maternal obesity and offspring obesity (Si et al., 2023). Previous studies conducted in high-income countries have also revealed that maternal overweight/obesity before (Brix et al., 2019) and during pregnancy (Kubo et al., 2018) were associated with offspring earlier onset of pubertal milestones including Tanner stages, voice break, first ejaculation and menarche.

Besides the pathway through childhood overweight/obesity, it has been suggested that maternal pre-pregnancy overweight/obesity can promote in-utero hyperinsulinemia (Catalano et al., 2009). Insulin can increase gonadotropin releasing hormone (GnRH) and LH that is upstream to sex hormone secretion (Wolfe et al., 2014). This may explain offspring early pubertal timing separate from the childhood overweight/obesity pathway. Previous studies on maternal overweight/obesity and offspring pubertal timing focused on Tanner staging (Walvoord, 2010) and age at menarche (Karapanou and Papadimitriou, 2010). While these are commonly used measurements (Walvoord, 2010), APHV is also considered to be a dependable measurement that can be used for both sexes (Chen et al., 2021; Tanner et al., 1966). Our study contributes to the literature by using APHV to measure pubertal timing, allowing the examination of how maternal pre-pregnancy overweight/obesity impacts puberty in both males and females.

Maternal race/ethnicity and early offspring pubertal timing

Non-Hispanic Black race/ethnicity may be associated with earlier pubertal timing (Freedman et al., 2002; Kaplowitz et al., 2001). While the mechanisms have yet been revealed, some pathways have been suggested. Firstly, genetic factors have been associated with non-Hispanic Black race/ethnicity and metabolism (Brown and Walker, 2016). One pathways may be through higher leptin and higher insulin levels among non-Hispanic Black individuals compared to non-Hispanic White individuals (which persisted after adjusting for fat mass) (Wong et al., 1998). Secondly, race/ethnicity may be a proxy for adverse experiences that lead to earlier pubertal timing (Ellis and Garber, 2000; Karapanou and Papadimitriou, 2010; Katagiri et al., 2021). Family adversity including father’s absence, remarriage, family conflict are suggested to lead to earlier pubertal timing (Ellis and Garber, 2000; Katagiri et al., 2021). U.S. non-Hispanic Black individuals are known to face poverty due to high rate of single-mother-headed households (Eggebeen and Lichter, 1991). In our population, there were 385 (65.4%) non-married individuals among non-Hispanic Black individuals and 251 (68.4%) non-married individuals among individuals with other race/ethnicity including widowed, divorced, separated and single households. Maternal depression (Ellis and Garber, 2000), low parental education attainment and insecure employment (Karpati et al., 2002) are also suggested social factors leading to earlier offspring pubertal timing. It has also been suggested that non-Hispanic Black individuals are more likely to experience chronic and severe depression compared to non-Hispanic White individuals, due to racial discrimination and low socioeconomic status (Bailey et al., 2019). Non-Hispanic Black children may be at higher risk of earlier pubertal timing due to social context.

Race/ethnicity and heavy metal exposures

In this study, we found that non-Hispanic Black participants were more likely to be exposed to Pb and Cd. Exposure to heavy metals such as Pb, Cd, Hg can occur in daily life and are associated with behaviors such as dietary intake, drinking water, smoking and exposure to industrial pollution and high-density traffic in urban environments (Iyengar and Rapp, 2001). Previous literature suggests that poverty and race/ethnicity may interact, increasing socioenvironmental vulnerability that leads to ingestion, inhalation and dermal exposure to heavy metals (Aelion et al., 2013; Jones et al., 2022). The well-known role of single-mother-headed households as a mediator between non-Hispanic Black race/ethnicity and poverty, did not fully explain the pathway in our study (Eggebeen and Lichter, 1991). Other possible pathways include non-Hispanic Black individuals having higher exposure to heavy metals due to vulnerable living and working conditions (Jones et al., 2022), disadvantaged socioeconomic status, and racial discrimination (Jones et al., 2022).

Maternal heavy metal exposure and early offspring pubertal timing

Our study suggested that heavy metals had modest impact on earlier APHV. Regarding Hg, this is in line with previous literature that revealed earlier APHV (Wang et al., 2021). Hg can stimulate the hypothalamus accelerating the HPG axis and sex hormones leading to earlier pubertal timing (Tan et al., 2009). Regarding Cd, its influence on pubertal timing remains controversial. Our findings were consistent with research indicating that Cd exposure leads to earlier pubertal timing through an estrogen-mimicking function (Johnson et al., 2003; Stoica et al., 2000). On the other hand, Cd may decrease testosterone, estradiol and progesterone levels leading to delayed pubertal timing (Interdonato et al., 2015; Samuel et al., 2011). Pb function also remains controversial. A previous study indicated that Pb contributes to delaying puberty (Dearth et al., 2002) through contributing to the suppression of gonadotropin and sex hormones (Sokol et al., 1985; Vivoli et al., 1993). However, a review paper revealed that short-term exposure to Pb leads to increased gonadotropin and testosterone (Doumouchtsis et al., 2009).

In our study, maternal heavy metals were measured 24-72 hours after delivery, reflecting heavy metal concentrations during the third trimester of pregnancy. Previous literature indicates that high Cd and Pb concentrations measured from blood samples during the first trimester of pregnancy were not significantly associated with menarche (Malin Igra et al., 2023). A study that focused on Pb exposure by trimester of pregnancy revealed that only high Pb concentration, measured from blood samples, during the second trimester of pregnancy were significantly associated with delayed menarche (Jansen et al., 2018). Authors suggest that the second trimester may be a sensitive period as this is when structural differences of the hypothalamus and immunoreactivity of estrogen receptors on the hypothalamus show differences among sexes, indicating that hypothalamus function may differ between sexes (Jansen et al., 2018). The association we observed between heavy metal exposures during the third trimester of pregnancy and earlier APHV could be due to an accumulation of heavy metals during the second and third trimesters (Gwaltney-Brant, 2013). It is also worth noting that the direction of which pubertal timing shifted differed between our study and prior study (Jansen et al., 2018), which may reflect lack of consensus about the physiological role lead plays in pubertal maturation (Doumouchtsis et al., 2009; Sokol et al., 1985; Vivoli et al., 1993).

Joint exposure to maternal pre-pregnancy overweight/obesity and heavy metal concentration in relation to offspring pubertal timing

Among females, maternal pre-pregnancy overweight/obesity was not associated with maternal Cd concentration during pregnancy, and Cd exposure itself did not have a strong association with APHV. However, exposure to both maternal pre-pregnancy overweight/obesity and high Cd concentration showed a joint association with earlier APHV in females, and the coefficient was close to the sum of the individual coefficients of maternal pre-pregnancy overweight/obesity and Cd exposure alone. Among males, maternal pre-pregnancy overweight/obesity was not associated with maternal Pb concentration during pregnancy, and Pb exposure alone did not have a significant association with APHV. However, exposure to both maternal pre-pregnancy overweight/obesity and high Pb concentration showed a joint association with earlier APHV in males, and the coefficient was close to the sum of the individual coefficients of maternal pre-pregnancy overweight/obesity and Pb exposure alone. This finding indicates a joint impact of maternal pre-pregnancy overweight/obesity and high concentrations of Cd and Pb during pregnancy on offspring pubertal timing. This result aligns with previous literature that suggests a joint impact of maternal pre-pregnancy overweight/obesity and toxic heavy metals on childhood metabolism (Huang et al., 2022). In our study, maternal pre-pregnancy overweight/obesity was not associated with heavy metal exposure. Our findings highlight that when pre-pregnancy overweight/obesity and Cd or Pb exposure occur simultaneously, there may be a joint impact on offspring pubertal timing. Therefore, careful attention should be paid to Cd and Pb exposure in pregnant women, especially those who experienced overweight/obesity before pregnancy.

Joint exposure to race/ethnicity and heavy metal concentration in relation to offspring pubertal timing

In this study, non-Hispanic Black race/ethnicity was significantly associated with higher Pb and Cd concentrations, aligning with previous research that revealed differential exposure burdens of heavy metals by race/ethnicity (Davis et al., 2014; Mijal and Holzman, 2010). Pregnant women are exposed to Pb, Cd, Hg through dietary intake, drinking water, smoking, industrial pollution and heavy-density traffic in urban environments (Iyengar and Rapp, 2001). Non-Hispanic Black individuals may face greater heavy metal exposure due to low SES or poverty (Davis et al., 2014). Among males, joint exposure to non-Hispanic Black race/ethnicity and high Pb concentration had pronounced association with earlier APHV. The coefficient of the joint association of non-Hispanic Black race/ethnicity and high Pb concentration with APHV was close to the sum of the individual coefficients of non-Hispanic Black race/ethnicity and Pb exposure alone. This suggests a joint association of non-Hispanic Black race/ethnicity and Pb exposure on earlier APHV among males. These findings imply that the non-Hispanic Black population may be more prone to earlier pubertal timing due to the joint associations of race/ethnicity and heavy metal concentrations.

Explanation of the difference between sexes

Previous research on in-utero metal exposure and pubertal timing has predominantly focused on females (Jansen et al., 2018; Malin Igra et al., 2023; Wang et al., 2021). This study is the first to examine these associations in both sexes. Among the BBC participants, co-exposures to Hg, Pb, and Cd modestly decreased APHV in males, but not females. This finding suggests that puberal development in males is sensitive to multiple heavy metal co-exposure. Although concrete biological pathways have yet to be identified, basic research in animals indicates that in-utero exposure to environmental toxic metals is more likely to impact pubertal growth in males (Ronis et al., 1996). Another key point is that females appeared more susceptible to Cd exposure, while males were more affected by Pb exposure. One possible explanation for this difference is that Cd has estrogen-mimicking properties, which may make females more vulnerable to Cd exposure (Malin Igra et al., 2023). In contrast, Pb may lead to short-term increases in gonadotropin and testosterone (Doumouchtsis et al., 2009), which may make males more susceptible to Pb exposure. Further research on males is needed to confirm these sex differences.

Policy implications, clinical implications, and future research

This research underscores the need for careful monitoring of heavy metal exposures during pregnancy, particularly for women who are overweight/obese and who are non-Hispanic Black. Non-Hispanic Black individuals are at greater risk of heavy metal exposures, which may contribute to the higher risk of early pubertal timing, especially given the joint associations of race/ethnicity and heavy metal concentrations. Socioeconomic disadvantage plays a multidimensional role between race/ethnicity and heavy metal exposure, necessitating targeted support and interventions. Future research is needed that examines in-utero heavy metal exposure using umbilical cord blood, which may provide deeper insights into fetal exposure levels. Additionally, there is a strong need for more research on pubertal timing in males, as this area remains underexplored.

Limitations

Our study had some limitations. First, information on race/ethnicity was self-reported by mothers. The race/ethnicity category in our study focused on a social identity (rather than on biological or genetic indicators) such that some biological African descendants may have not been categorized in the non-Hispanic Black population. Future studies examining both social factors and indicators of genetic race/ethnicity may further elucidate how race/ethnicity and genetics are involved in pubertal acceleration.

Second, our study was not able to reveal the sensitive periods of heavy metal exposure regarding offspring pubertal maturation. Previous literature suggests that the second trimester may be a critical period because this is when sex differentiation occurs in the hypothalamus(Jansen et al., 2018). Future research collecting samples longitudinally through the first, second and third trimester of pregnancy may add further insights regarding sensitive periods for exposure to heavy metals.

Third, while information on maternal occupation would have added further insights to our research, the BBC did not have such information. We were able to consider maternal education attainment as a social confounder, however maternal occupation could have provided additional insights to maternal heavy metal exposure pathways. Additionally, neighborhood-level heavy metal concentrations would also reflect ingestion, inhalation and dermal exposure to heavy metal (Iyengar and Rapp, 2001), which could be studied in future research.

Fourth, we were not able to include economic factors in our analysis. Although the BBC data collected information on household income, we were not able to include it in our analysis due to high invalid values (N=464, 48.5%) (Che et al., 2023). Previous literature suggests that poverty may be a pathway to both higher heavy metal exposure (Jones et al., 2022) and earlier pubertal timing (Belsky et al., 1991). Future research considering poverty and how it contributes to the pathway between environmental toxicity and pubertal timing can complement our research.

Strengths

Strengths of this study should also be highlighted. Firstly, the BBC is a predominantly Black population, permitting a race-specific analysis of a marginalized understudied population (Pearson et al., 2022). The longitudinal study design enabled us to reveal the temporal relation between fetal exposures and adolescent outcomes.

Secondly, a large number of repeated height measurements conducted in a medical setting were available from infancy to late adolescence. These data permitted the calculation of APHV allowing us to capture pubertal growth in both sexes (Chen et al., 2021; Tanner et al., 1966).

Thirdly, application of a mixture analysis using BKMR model enabled us to consider the associations of heavy metal mixtures (Bobb et al., 2015). This method has practical significance as individuals are often exposed to multiple heavy metals in real life (Gwaltney-Brant, 2013).

Conclusion

Our study showed that maternal pre-pregnancy overweight/obesity and non-Hispanic Black race/ethnicity were both associated with earlier APHV in both sexes. While heavy metal exposure alone showed a weak association with APHV, the joint associations of heavy metal concentrations and maternal pre-pregnancy overweight/obesity, as well as heavy metal concentrations and race/ethnicity, significantly accelerated APHV. Among the heavy metals, Cd was most notable for its impact in females, while Pb had the most significant association in males. Co-exposure to Hg, Pb, and Cd modestly decreased the APHV in males, but not females.

Abbreviations:

BMI

body mass index

BBC

Boston Birth Cohort

APHV

age at peak height velocity

HPA

hypothalamic-pituitary-adrenal

HPG

hypothalamic-pituitary-gonadal

EDC

endocrine-disrupting chemical

Hg

mercury

Cd

cadmium

Pb

lead

LH

luteinizing hormone

LOD

limit of detection

IQR

interquartile range

SES

socioeconomic status

BKMR

Bayesian kernel machine regression

Data availability

The data, data dictionary, and analytical programs for this manuscript are not currently available to the public. However, they can be made available upon reasonable request and after the review and approval of the institutional review board.