Compared with those who were followed, the mothers lost to follow-up were younger, less educated, shorter, and poor; more were recruited at the Ministry of Health hospital (). A higher proportion of mothers lost to follow-up reported having no prenatal care, absence of the child’s father, and fathers who smoked. Finally, their median levels of DDE and DDT were lower compared with the followed mothers. Comparisons between those with one and two or more follow-up visits are presented in . Those with only one visit had lower median levels of DDE and DDT.
Overall, the mothers in this study had little education, were young, urban-dwelling and poor or very poor, and most had previously had a child; few reported smoking (n=9) or drinking alcohol (n=23) during pregnancy (). The fathers of the children were older than the mothers and a higher proportion of them had more than 12 years of school (Table 1a
of supplementary data). The percentage of breastfed children was high, the duration of any breastfeeding was long, and the duration of exclusive breastfeeding was short ().
Because of the exclusion criteria for the original study, the children were delivered at term with birth weights in the normal range (Tables and ). Anthropometry at birth compared with the 2000 CDC growth reference (Kuczmarski et al., 2000
) showed that length for age was on average −0.15 SD (quartiles −0.56 and 0.38) below the mean for US boys at birth, and weight for age was on average −0.38 SD (quartiles −0.88 and 0.04) below the mean for US boys at birth (data not shown).
Characteristics of the Boys by Prenatal Exposure to DDE, Tapachula, Chiapas, Mexico 2002-2005
Across maternal serum levels of DDE, anthropometric measurements of the children at birth were similar as were the total number of follow-up visits and the interval between them. However, an increase in the median of children’s age at first and last follow-up visits with levels of DDE was observed, although the difference in ages were on average less than three months ().
Higher levels of prenatal DDE exposure showed no crude associations with height or BMI SDS (). With height in its original units (cm), no pattern was observed across categories of DDE at any age (). Although after 24 months of age the figure suggests a small effect with higher DDE, the numbers were sparse at those ages. Similarly, no clear pattern of any effect due to DDE exposure on BMI (kg/m2) was observed; at 5 months of age children in the higher exposure categories of DDE had lower BMI than those in low exposure categories () but again the data were sparse in this age range.
Mean of height (cm) of the boys during the study period by prenatal exposure to p,p’-DDE (μg/g lipids), Tapachula, Chiapas, Mexico 2002-2005
Mean of body mass index (kg/m2) of the boys during the study period by prenatal exposure to p,p’-DDE (μg/g lipids), Tapachula, Mexico 2002-2005
Unadjusted and adjusted results from the multivariate models of height SDS generally showed no statistically significant associations with higher levels of DDE, except for the interaction term in the second category of exposure (). The adjusted coefficients for DDE from the model of height can be interpreted as follows. At 15.7 months of age (median age at first follow-up) the main effect term indicates that, compared to the least exposed group (DDE: < 3.01 μg/g), children in the second exposure category (DDE: 3.01-6.00 μg/g) were shorter by 0.121 SDS; the interaction term however, indicates that their rate of growth was slightly increased by 0.019 SDS a month. Thus, the observed difference in height SDS was reduced until age ~22 months when these children had similar height SDS as those from the referent group (). At 15.7 months of age, children in the third exposure category (DDE: 6.01-9.00 μg/g) compared to the referent group were shorter by 0.03 SDS and their rate of growth was decreased by −0.005 a month; these children continued to grow at a slower rate and the observed difference in height SDS increased with time (). Unexpectedly, at 15.7 months of age, children with the highest exposure (DDE: >9.00 μg/g) compared to the least exposed were taller by 0.069 SDS (equivalent to +0.21 cm) and their rate of growth was slightly increased by 0.01 SDS a month (equivalent to +0.03 cm). Thus, the difference in height SDS increased with time and they grew taller than the referent group (). The observed differences in height before and after age 15.7 months were not significant when comparing the least exposed group with the third (DDE: 6.01-9.00 μg/g) and the highest exposure categories (DDE: >9.00 μg/g); however for the second category (DDE: 3.01-6.00 μg/g) the differences were significant up to 10 months of age and at 38 months (, table 2b
of supplementary data).
Coefficients From the Multilevel Model for Change of Height (SDSa) in Boys From Tapachula, Chiapas, Mexico 2002-2005 (n=788 boys, 2,633 observations)
Estimated height (SDS)a of the boys at different ages by prenatal exposure to p,p’-DDE (μg/g lipids), Tapachula, Mexico 2002-2005
After adjustment, predictors of height SDS at 15.7 months of age were child’s age at measurement (taller), hospital of recruitment (shorter at Ministry of Health), and maternal education (taller) and height (taller). Rural residence was the only predictor (decreased growth) of the monthly rate of growth in the height SDS model ().
For BMI SDS (), no statistically significant associations with DDE emerged in the unadjusted and adjusted models. The adjusted coefficients for DDE from the model of BMI are interpreted similarly as those for height. At 15.7 months of age the main effect term indicates that, compared with children in the lowest exposure category, those with the highest exposure had a BMI higher by 0.073 SDS (equivalent to +0.10 kg/m2
); the interaction term indicates that they had a slower rate of weight gain by −0.002 a month (equivalent to −0.003 kg/m2
). Therefore, the difference in BMI SDS decreased with time and they had similar BMI as those from the referent group by age 38 months (). The observed differences in BMI before and after age 15.7 months were not significant when comparing the least exposed group with any of the higher exposure categories (, table 3b
of supplementary data).
Coefficients From the Multilevel Model for Change of Body Mass Index (SDSa) in Boys From Tapachula, Chiapas, Mexico 2002-2005 (n=787 boys, 2,632 observations)
Estimated body mass index (SDS)a of the boys at different ages by prenatal exposure to p,p’-DDE (μg/g lipids), Tapachula, Mexico 2002-2005
Predictors for BMI SDS at 15.7 months of age after adjustment were: having a mother who smoked during pregnancy (higher BMI), who had no education (lower BMI), and obesity before pregnancy (higher BMI). Years of maternal education was the only predictor (higher BMI) of the average rate of growth in the BMI SDS model.
Evidence of effect modification by duration of breastfeeding or maternal smoking (having ever smoked >100 cigarettes) was not observed in models of either height or BMI SDS. Using DDT as the exposure variable in multivariate models did not show any statistically significant associations with height or BMI SDS (data not shown).
Modeling height and BMI in their original units confirmed the results observed in Tables and . When the age-specific z-scores of height and weight, and the height-specific SDS (and z-scores) of weight were modeled as outcomes, no associations with DDE were seen (data not shown). Stratifying the BMI and weight for height (SDS) models by child’s age (at < 24 and ≥ 24 months) showed similar results; no associations with DDE were observed in either group (data not shown). The results shown in Tables and were not materially different after a) adjustment for anthropometry at birth, b) excluding those with only one follow-up visit, c) stratifying by number of follow-up visits (<3 and ≥ 3), d) restricting the analysis by age (at ≤ 30 or ≤ 25 months), e) using deciles of DDE exposure, or f) dividing the lowest category of DDE into finer categories. Removing gestational age from the final models showed essentially the same results as in Tables and (data not shown).