We found that higher concentrations of PBDEs in maternal serum during pregnancy were associated with lower birth weight in a population of low-income women living in California. Each 10-fold increase in concentrations of BDE-47, -99, and -100 was associated with an approximately 115-g decrease in birth weight. This association was relatively large; for comparison, the difference in birth weight observed between smokers and nonsmokers is between 150 and 250 g (37
). When maternal weight gain or thyroid-stimulating hormone levels were added to the models, the association of PBDEs and birth weight was slightly reduced and no longer statistically significant. No statistically significant associations were seen with birth length and head circumference, although the direction of the association was consistently negative.
Our findings of an association of PBDEs and birth weight are consistent with those of some recent studies. For example, Wu et al. (24
) found that levels of BDE-28, -47, -99, -153, and -183 were higher in the cord blood of infants with adverse birth outcomes, and Chao et al. (25
) found that BDE-47, -99, -100, and -209 were higher in breast milk of mothers delivering lower birth weight infants. However, both of these studies were case-control studies with a very small number of low-birth-weight cases (n
= 25 and 10, respectively). Two other small human studies have found no association of PBDE exposure during pregnancy and fetal growth (26
). Median PBDE concentrations in all 4 previous studies were comparable to or lower than what we observed. The present study, with its sample size of 286 infants, is the first prospective study to examine this question and is the largest study to date.
Although animal studies have not found PBDEs to be associated with birth weight, studies have found decreased weight gain of offspring in the early postnatal period, which is comparable to the third trimester of development in humans. Kodavanti et al. (5
) found that exposing pregnant rats to 10 and 30 mg/kg/day of DE-71, a commercial penta-BDE mixture, was associated with lower weight gain and smaller body weights in female (but not male) offspring by postnatal day 29. Kim et al. (38
) found lower pregnancy weight gain in pregnant rats dosed with BDE-209 and lower weight gain in male (but not female) offspring by postnatal day 28. However, it is difficult to tell whether the doses that caused body weight effects in rats are environmentally relevant in humans. Other animal studies found no associations of maternal PBDE exposure with offspring's birth weight or body weight gain (13
The findings of this study are limited primarily to the components of the penta-BDE mixture. Because these congeners are highly correlated in our population (36
), we are limited in our ability to distinguish which specific congeners may be causally associated with birth weight. An additional limitation of this study is that we were unable to measure BDE-209, the main constituent of the deca-BDE mixture. Thus, this study cannot provide any information on the association of the deca mixture with birth outcomes.
A large number of women in the study population had insufficient serum volume for analysis of PBDEs, which may result in selection bias. The women who were not included in the analysis had lived in the United States for slightly more years on average than those who were included. Years of residence in the United States is positively correlated with PBDE concentrations in this population, so it is possible that we excluded women with higher PBDE levels. Although birth in the United States was associated with lower birth weight in this population, total years living in the United States among immigrants was not. However, it is possible that the results may have been different if the entire population had been included.
An additional limitation is that, because PBDEs were measured in blood collected during pregnancy, pregnancy- and weight gain-related pharmacokinetics could influence exposure measurements. We considered that pregnancy weight gain might be on the causal pathway and did not include it in our main models. However, weight gain could also be a confounder if increased weight diluted PBDE measurements on a lipid basis. Controlling for maternal weight gain in our analysis diminished the magnitude of the effect on birth weight, but the negative association remained (P values ranging from 0.07 to 0.09 for BDE-47, -99, -100, and ΣPBDEs).
Despite these limitations, this study benefits from a strong prospective design, with PBDE concentrations measured in maternal serum collected at a relevant exposure time point (near the start of the third trimester of pregnancy), and information on birth outcomes collected by hospital staff blinded to PBDE exposure status. The study population is relatively homogeneous with regard to race, socioeconomic status, and time residing in the United States, but we were also able to control for several potential confounders.
The study population comprised mainly low-income, Mexican immigrant women living in an agricultural region of California. As such, the results may not be generalizable to the US population. Mexican immigrant women in the United States have been shown to have very low rates of low birth weight (41
), and this was reflected in our study population (the proportion of low birth weight births in the United States was 8% in 2000 compared with 3% in this study). It should be noted that levels of PBDEs in our study were also lower than those in the general US population. It is possible that, among women with higher PBDE exposures and greater risk of low birth weight, the impact of PBDEs would be of greater clinical significance.