These findings provide the first biomarker-based evidence that maternal exposure to insecticides is associated with autism among offspring. Although further research is necessary to replicate this finding, this study has implications for the prevention of autism and may provide a better understanding of its pathogenesis.

The odds of autism among offspring were significantly increased with maternal p,p′-DDE levels that were in the highest 75th percentile, with adjustment for maternal age, parity, and history of psychiatric disorders (odds ratio=1.32, 95% CI=1.02, 1.71). The odds of autism with intellectual disability were increased by greater than twofold with maternal p,p′-DDE levels above this threshold (odds ratio=2.21, 95% CI=1.32, 3.69). There was no association between total levels of maternal PCBs and autism.

The investigation was derived from the Finnish Prenatal Study of Autism, a national birth cohort study based on a nested case-control design. Cases of autism among children born between 1987 and 2005 were ascertained by national registry linkages. In cases of childhood autism and matched control subjects (778 matched case-control pairs), maternal serum specimens from early pregnancy were assayed for levels of p,p′-DDE and total levels of PCBs.

Autism is a complex neurodevelopmental disorder with a largely unknown etiology. To date, few studies have investigated prenatal exposure to toxins and risk of autism by using maternal biomarkers of exposure. Persistent organic pollutants are lipophilic halogenated organic compounds and include the insecticide dichlorodiphenyltrichloroethane (DDT), as well as its metabolite p,p′-dichlorodiphenyl dichloroethylene (p,p′-DDE), and polychlorinated biphenyls (PCBs). The objective of this study was to test whether elevated maternal levels of persistent organic pollutants are associated with autism among offspring.

Autism is a complex neurodevelopmental disorder with a largely unknown etiology. It is characterized by impaired language, disrupted reciprocal social interactions, and stereotyped behaviors and interests (1). Both genetic and environmental factors have been associated with the disorder (2–4). To date, few studies have examined an association between prenatal exposure to toxins and autism, and among these, most have been based on ecologically, rather than serologically, documented exposures. For example, specific associations were reported for residential proximity to sites contaminated with pesticides (5, 6) and traffic-related air pollution (7). However, associations between prenatal exposure to air pollution and risk of autism spectrum disorder were not shown in a Swedish twin study (8) or in a large collaborative European study (9).

Persistent organic pollutants are lipophilic halogenated organic compounds and include the insecticide dichlorodiphenyltrichloroethane (DDT), as well as its metabolite p,p′-dichlorodiphenyl dichloroethylene (p,p′-DDE), and polychlorinated biphenyls (PCBs) (10). Before the late 1970s, these compounds were in widespread use in developed countries (mainly as insecticides [i.e., DDT] and in transformers and electrical equipment [i.e., PCBs]). Although these chemicals were widely banned in many countries more than 30 years ago, they became ubiquitous in these countries, including in the United States (11) and Finland (12). Because of their lipophilic nature and chemical half-lives of as long as several decades (13), these compounds persist in the food chain, particularly through fatty food sources, resulting in continuous exposure among populations. Persistent organic pollutants are transferred across the placenta, resulting in lipid-adjusted cord blood concentrations that range from 30% to 50% of levels found in maternal plasma (14). Thus, there is ongoing prenatal exposure potential among nearly all children because of existing maternal body burdens (11), as demonstrated in a nationally representative sample of U.S. women in 2003–2004 (11).

Maternal exposure to persistent organic pollutants has been associated with aberrant perinatal outcomes and childhood neurocognitive outcomes. Increasing maternal levels of p,p′-DDE were associated with an elevated risk of preterm birth in a study with a large sample size (15), and reductions in indices of psychomotor development and other cognitive functions (16, 17) as well as delayed processing speed (18) have been observed in exposed offspring. Furthermore, maternal PCBs have been associated with aberrant neurocognitive outcomes, although findings are inconsistent (for a review, see reference 19).

Although associations with autism-related behaviors have been reported in several studies that used interview and ecologic data on maternal exposure to persistent organic pollutants (5, 6, 20, 21), few studies have examined biomarker-based measures of maternal exposure to these pollutants and autism spectrum disorder in offspring (22, 23). In a small pilot study from the Finnish Prenatal Study of Autism, we previously reported that maternal levels of p,p′-DDE and PCBs were associated with autism in childhood, although the findings were not statistically significant (22). In the Early Markers of Autism study, Lyall et al. (23) observed increased mean levels of several PCB congeners, including PCB 138/158 and PCB 153, in mothers of children with autism spectrum disorder.

We therefore hypothesized that maternal p,p′-DDE levels and total PCB levels, each in the highest quartile of the distribution, would be related to risk of autism among offspring. Supplementary analyses were conducted to investigate whether offspring sex and comorbid intellectual disability modified the relationship between maternal exposure to persistent organic pollutants and autism.

Method

Study Population

The study is derived from a large national population-based birth cohort. The Finnish Prenatal Study of Autism is based on a nested case-control design. The sampling frame was defined such that all members of the national birth cohort were within the age of risk of autism. Toward this end, the study subjects comprised all offspring born in Finland from 1987 to 2005, and they were followed up until 2007. The study methods are described in further detail by Lampi et al. (24).

Description of the Birth Cohort, Biobank, and National Registries

All offspring in the Finnish Prenatal Study of Autism were derived from the Finnish Maternity Cohort, which consists of more than 1 million pregnancies with archived prenatal serum specimens drawn since 1983 (24). Sera were obtained during the first trimester and early second trimester (months 2–4 of pregnancy) from more than 98% of pregnant women in Finland. One maternal serum sample was acquired for each pregnancy. After the screening, serum samples were stored as one aliquot at –25°C in a single biorepository at the National Institute for Health and Welfare in Oulu, Finland. All samples in the Finnish Maternity Cohort can be linked to offspring by a unique personal identification number, assigned to all residents of Finland since 1971.

Identification of Case and Comparison Subjects

The Finnish Hospital and Outpatient Discharge Register was used to identify all recorded diagnoses from psychiatric hospital admissions and outpatient visits for childhood autism (ICD-10 code F84.0) among individuals registered with the Finnish Maternity Cohort. Registry diagnoses of childhood autism were validated with the Autism Diagnostic Interview–Revised (24). Computerized data are available from January 1, 1987, to the present. Only singleton births were included. Cases diagnosed over the sampling frame were identified from registry linkages between the Finnish Maternity Cohort and the Finnish Hospital and Outpatient Discharge Register from January 1, 1987, to December 31, 2007. The total number of childhood autism cases in the Finnish Prenatal Study of Autism was 1,132.

Case subjects were matched 1:1 to comparison subjects (singleton births only) on date of birth, sex, birthplace, and residence in Finland. Comparison subjects were drawn from the birth cohort and were without a diagnosis of autism spectrum disorder (no ICD-10 code F84.0 diagnosis). The analytic sample comprised 778 case subjects (from the 1,132 total cases of autism spectrum disorder mentioned above) and 778 matched control subjects.

Laboratory Analyses

All assays were performed blind to case-control status. Matched case and control subjects were analyzed in the same run to minimize variation between runs. In the analysis of persistent organic pollutants, we tested the following two primary hypotheses: autism in offspring would be associated with increased maternal concentration of p,p′-DDE and autism in offspring would be associated with increased maternal concentration of total PCBs. The analytical method used is described in detail elsewhere (25). Briefly, ethanol and [13C]-labeled internal standards of each persistent organic pollutant compound were added to the serum samples. Dichloromethane-hexane was added for extraction of persistent organic pollutants, followed by activated silica to bind the sample water, ethanol, and protein precipitate. The upper dichloromethane-hexane layer was poured into a multilayer silica column for the removal of coextracted compounds that interfere with the gas chromatography-mass spectrometry quantitation. Persistent organic pollutants were eluted from the cleanup column with additional dichloromethane-hexane. The recovery standard [13C]-PCB-128 was added, and the eluate was concentrated for gas chromatography with tandem mass spectrometry analysis for quantification of persistent organic pollutants.

For each batch of samples, a control serum sample from the National Institute of Standards and Technology, Standard Reference Material 1958, was included. Recoveries of p,p′-DDE and PCB levels varied from 86% to 106% (coefficient of variation, 1.6%−6.5%) of the certified concentrations for Standard Reference Material 1589 and from 83% to 101% (coefficient of variation, 2.0%−6.6%) of the calculated concentrations for diluted Standard Reference Material 1589, respectively. The limits of quantification were 5 pg/mL for each PCB congener and 40 pg/mL for p,p′-DDE. Fresh weight serum concentrations of persistent organic pollutants, which demonstrated high correlation with lipid-based concentrations in a previous study (overall, r=0.95) (26), are reported here.

Classification of Persistent Organic Pollutant Variables

In order to limit the number of analyses of the compounds, we focused on two hypothesized primary measures of maternal exposure to persistent organic pollutants: maternal p,p′-DDE levels in the highest 75th percentile of the distribution and maternal total PCBs, quantified as the sum of concentrations of the 10 measured congeners (PCB 74, PCB 99, PCB 118, PCB 138, PCB 153, PCB 156, PCB 170, PCB 180, PCB 183, and PCB 187), in the highest 75th percentile of the distribution. These PCBs were selected because they represent approximately 85%−90% of all PCBs on a mass basis.

The study was approved by the ethical committees of the hospital district of Southwest Finland and the Finland National Institute for Health and Welfare as well as the institutional review board of the New York State Psychiatric Institute. At the time all maternal serum specimens were obtained, mothers provided informed consent after receiving a description of the nature and possible consequences of the procedure and the data derived from serum analyses.

Statistical Analysis

We computed descriptive statistics and correlations between levels of persistent organic pollutants. These were analyzed separately for case and control subjects. Potential confounders were selected on the basis of previous relationships with exposure to persistent organic pollutants or autism from other studies (27) and compared between the two study groups by using chi-square and t tests. Potential confounders were maternal age, number of previous births (0 or ≥1), socioeconomic status (upper white collar, lower white collar, blue collar, or other), maternal and parental history of psychiatric disorders, and gestational week of the blood draw (Table 1). Data on maternal age, maternal socioeconomic status, and previous births were acquired from the Finnish Medical Birth Register. Data on maternal and paternal history of psychiatric disorders were acquired from the Finnish Hospital and Outpatient Discharge Register. Data on paternal age were obtained from the Finnish Population Register. Data on gestational week of the blood draw were obtained from the Finnish Maternity Cohort.

TABLE 1. Comparison of Covariate Distributions Among Case Subjects With Autism and Matched Control Subjects Covariate Case Subjects (N=778) Control Subjects (N=778) N % N % pa Birth year 1.0 1987–1990 68 8.7 68 8.7 1991–1993 209 26.9 209 26.9 1994–1996 208 26.7 208 26.7 1997–2005 293 37.7 293 37.7 Offspring sex 1.0 Male 606 77.9 606 77.9 Female 172 22.1 172 22.1 Maternal age (years) 0.01 ≤19 9 1.2 21 2.7 20–24 107 13.8 134 17.2 25–29 257 33.0 254 32.7 30–34 228 29.3 233 30.0 35–39 141 18.1 116 14.9 ≥40 36 4.6 20 2.6 Maternal parityb 0.001 0 189 24.3 254 32.9 1 266 34.2 241 31.2 2 177 22.8 134 17.3 ≥3 145 18.7 144 18.6 Maternal socioeconomic statusc 0.46 Upper white collar 89 13.5 106 16.6 Lower white collar 305 46.3 284 44.4 Blue collar 134 20.3 131 20.5 Other 131 19.9 119 18.6 Maternal history of psychiatric disordersd 0.0003 No 650 83.6 699 89.9 Yes 128 16.5 79 10.2 Parental history of psychiatric disordersd 0.02 No 586 75.3 624 80.2 Yes 192 24.7 154 19.8 Mean SD Mean SD pe Gestational weeks at blood draw 10.9 3.47 10.8 3.4 0.49 TABLE 1. Comparison of Covariate Distributions Among Case Subjects With Autism and Matched Control Subjects Enlarge table

Appropriate to the case-control study design, point and interval estimates of odds ratios for the association of maternal levels of p,p′-DDE and total PCBs with autism were obtained by fitting conditional logistic regression models for matched sets. Statistical significance was set at a p value <0.05. Covariates were included in the adjusted models on the basis of associations with the outcome. We did not match on these covariates because of the disadvantages of overmatching (28) and because they could be controlled effectively in the multivariable analyses.

For the primary analyses of persistent organic pollutants (p,p′-DDE and total PCBs), exposures were analyzed as dichotomous variables, with cutoff points at the 75th percentile. Exploratory analyses were conducted after stratification by sex and intellectual disability of the case subjects, given well-known sex differences in autism (29) as well as extensive evidence of comorbid intellectual disability (30) (ICD-9 codes for intellectual disability: F317, F318.0, F318.1, F318.2, and F319; ICD-10 codes for intellectual disability: F70, F71, F72, F73, F78, and F79). Moreover, previous studies have indicated that some risk factors may be distinct for autism with intellectual disability (31) relative to autism without intellectual disability (32), including our previous finding that accelerated growth velocity of head circumference at 3 months of age was associated with autism with intellectual disability but not autism without intellectual disability (31). We examined effect modification of p,p′-DDE levels and total levels of PCBs by adding product terms to models for each variable by p,p′-DDE or by PCB levels higher than the 75th percentile. The evidence for heterogeneity of the odds ratios between strata for each potential effect modifier was assessed on the basis of the p values for the product terms. In order to evaluate whether maternal levels of PCB 138/158 and PCB 153 were associated with autism, we conducted supplementary analyses of these maternal PCBs and autism.

Statistical analyses were performed with SAS 9.4 (SAS Institute, Cary, N.C.). Bonferroni correction was not performed given that only two primary hypothesized variables were tested (maternal p,p′-DDE levels higher than the 75th percentile and maternal total PCB levels higher than the 75th percentile), as mentioned above.

Results

Covariates

Older maternal age, increased maternal parity, and maternal and parental history of psychiatric disorders were significantly associated with odds of autism in offspring (Table 1). There were no relationships between maternal socioeconomic status and odds of autism. Earlier birth year, older maternal age, lower maternal parity, and higher maternal socioeconomic status were associated with higher maternal levels of total PCBs and of p,p′-DDE among control subjects. Offspring sex and maternal or parental history of psychiatric disorders were not associated with maternal levels of persistent organic pollutants.

Primary Findings

The mean and median levels of maternal persistent organic pollutants among the study subjects are presented in Table 2. In almost all samples, p,p′-DDE was measured above the limits of quantitation (case and control subjects, N=775/778). Each of the PCB congeners was measured above the limits of quantitation in 95%–100% of samples. The mean maternal p,p′-DDE level among case subjects was 1,032 pg/mL (SD=2,176), compared with 811 pg/mL (SD=1,660) in corresponding control subjects. Additionally, the median levels of maternal p,p′-DDE were higher among case subjects (512 pg/mL [interquartile range, 263–948]) compared with control subjects (469 pg/mL [interquartile range, 249–806]). The mean level of total maternal PCBs was 1,022 pg/mL (SD=649) among case subjects, compared with 999 pg/mL (SD=660) among control subjects, and the median levels of total maternal PCBs were 884 pg/mL (interquartile range, 583–1,282) and 865 pg/mL (interquartile range, 570–1,258) among case and control subjects, respectively. For descriptive purposes, we have reported the mean and median levels of the 10 PCB congeners.

TABLE 2. Mean and Median Maternal Levels of Persistent Organic Pollutants (pg/mL) by Autism Case-Control Status Among Offspring Case Subjects (N=778) Control Subjects (N=778) Variable Mean SD Median Mean SD Median p,p′-Dichlorodiphenyl dichloroethylene 1,032.0 2,175.5 512.0 811.2 1659.8 469.4 Total polychlorinated biphenyls (PCBs)a 1022.4 649.4 883.6 998.6 659.7 864.8 PCB congener PCB 74 23.9 26.2 18.7 22.2 20.2 17.8 PCB 99 28.1 21.0 23.4 28.2 27.1 22.3 PCB 118 57.4 42.6 46.9 54.5 43.7 44.4 PCB 138 198.2 127.4 174.6 195.8 134.6 168.8 PCB 153 293.8 185.5 258.9 289.6 198.7 249.5 PCB 156 29.8 20.1 25.5 29.0 20.6 24.8 PCB 170 99.8 65.8 88.2 96.9 60.8 84.8 PCB 180 211.2 140.5 181.2 204.2 132.0 176.6 PCB 183 24.6 17.4 21.6 24.1 17.1 20.9 PCB 187 55.7 41.1 46.4 53.9 40.1 45.1 TABLE 2. Mean and Median Maternal Levels of Persistent Organic Pollutants (pg/mL) by Autism Case-Control Status Among Offspring Enlarge table

The odds of autism in offspring were significantly increased (by 32%) with maternal p,p′-DDE levels that were in the top 75th percentile of the control distribution (odds ratio=1.32, 95% CI=1.02, 1.71, p=0.03), with adjustment for maternal age, parity, and maternal history of psychiatric disorders (Table 3). For total maternal levels of PCBs, there was no increase in the adjusted odds of autism among offspring for exposure above the 75th percentile (odds ratio=0.95, 95% CI=0.73, 1.24, p=0.69). Adjusting models for maternal age and parity only or for maternal age, parity, and parental history of psychiatric disorders had minimal effect on the results.

TABLE 3. Association Between Maternal Levels of Persistent Organic Pollutants and Autism Among Offspring Case Subjects Control Subjects Unadjusted Adjusteda Persistent Organic Pollutant N % N % Odds Ratio 95% CI p Odds Ratio 95% CI p p,p′-Dichlorodiphenyl dichloroethylene >75th percentile 239 30.7 194 24.9 1.41 1.11, 1.80 0.01 1.32 1.02, 1.71 0.03 Total polychlorinated biphenyls (PCBs),b >75th percentile 202 26.0 194 24.9 1.06 0.84, 1.35 0.62 0.95 0.73, 1.24 0.69 TABLE 3. Association Between Maternal Levels of Persistent Organic Pollutants and Autism Among Offspring Enlarge table

Stratum-specific estimates for associations between maternal levels of p,p′-DDE and total maternal levels of PCBs and autism are summarized in Table 4. The association between maternal p,p′-DDE levels higher than the 75th percentile and odds of autism was significant among male offspring (odds ratio=1.35, 95% CI=1.02, 1.80, p=0.04) but not female offspring (odds ratio=1.19, 95% CI=0.67, 2.13, p=0.55), although the estimates for male compared with female offspring did not differ significantly (p value for interaction, 0.70). The increase in the odds of autism associated with maternal levels of p,p′-DDE higher than the 75th percentile was greater among case subjects with intellectual disability (odds ratio=2.21, 95% CI=1.32, 3.69, p=0.002) compared with case subjects without intellectual disability (odds ratio=1.22, 95% CI=0.88, 1.69, p=0.18), and these odds ratios were significantly different from one another (p value for interaction, 0.04). There were no associations between total maternal levels of PCBs and autism among male or female offspring and among case subjects with or without intellectual disability.

TABLE 4. Stratum-Specific Associations Between Maternal Levels of Persistent Organic Pollutants (>75th Percentile) and Autism Among Offspring Variable Case Subjects (N)a Control Subjects (N)a Odds Ratiob 95% CI p p-Intc p,p′-Dichlorodiphenyl dichloroethylene Sex 0.70 Male 605 601 1.35 1.02, 1.80 0.04 Female 172 172 1.19 0.67, 2.13 0.55 Intellectual disability in the case subject 0.04 Yes 218 216 2.21 1.32, 3.69 0.002 No 559 557 1.22 0.88, 1.69 0.18 Total polychlorinated biphenyls Sex 1.00 Male 605 601 0.95 0.70, 1.27 0.72 Female 172 172 0.95 0.54, 1.67 0.85 Intellectual disability in the case subject 0.93 Yes 218 216 0.97 0.60, 1.56 0.88 No 559 557 0.94 0.69, 1.28 0.70 TABLE 4. Stratum-Specific Associations Between Maternal Levels of Persistent Organic Pollutants (>75th Percentile) and Autism Among Offspring Enlarge table

We examined whether maternal levels of PCB 138 and PCB 153 were associated with autism, given the findings of Lyall et al. (23). No associations were observed for PCB 138 (odds ratio=0.90, 95% CI=0.60, 1.18, p=0.44) or PCB 153 (odds ratio=0.97, 95% CI=0.74, 1.26, p=0.80). Additionally, there were no associations between any of the other PCB congeners and autism.

Correlations Between Persistent Organic Pollutants

The correlation between maternal levels of p,p′-DDE and total PCBs was 0.4. Total PCBs were correlated with the levels of individual PCB congeners, with r values ranging from 0.74 to 0.99 (for further details, see Table S1 in the online supplement).

Discussion

This large national birth cohort study of maternal levels of persistent organic pollutants and autism among offspring produced two principal findings. First, maternal levels of p,p′-DDE were significantly increased in mothers of case subjects with autism compared with mothers of control subjects without autism. The findings persisted after adjustment for covariates related to autism. To our knowledge, this is the first biomarker-based evidence of this association. Second, maternal levels of total PCBs, and other PCBs variously defined, were unrelated to autism risk, and thus we did not replicate the associations between maternal levels of PCBs and autism in our previous pilot study from the Finnish Prenatal Study of Autism birth cohort, despite the similar source population and method. However, the sample size in the pilot study was small, and none of the findings reached statistical significance (22).

We propose two reasons for the observation that maternal exposure to p,p′-DDE was related to autism while maternal exposure to PCB was not. First, maternal exposure to DDT and DDE is associated with both premature birth and small gestational age status. Exposures to both of these compounds have been well replicated as risk factors for autism spectrum disorder (15, 33). In contrast, maternal PCB exposure has not been related to prematurity or small gestational age status (34). Second, p,p′-DDE inhibits androgen receptor binding, androgen-induced transcriptional activity, and androgen action, including in developing rats (35). Offspring of rats injected with valproic acid, an in utero risk factor for autism (36), exhibited reduced androgen receptor expression in most cerebellar lobules, in both male and female offspring (37); cerebellar abnormalities, including Purkinje cell numbers, have been observed in the brains of individuals with autism (38) and in rat offspring exposed prenatally to valproic acid (39). In contrast, PCBs increase androgen receptor transcription (40).

One possible reason for inconsistent findings of an association between maternal exposure to persistent organic pollutants and autism in offspring across studies is differences in the chemical mixtures and contexts of exposure between populations. A study that examined PCB 153 and p,p′-DDE levels in adults from four different geographic populations found that the correlation between the levels of these two persistent organic pollutants varied considerably, as did the associated covariates, likely reflecting differences in primary routes of exposure (41). Animal studies have demonstrated interactive effects on behavior and learning between different neurotoxicant chemicals, including PCBs and methylmercury (42). Therefore, it is possible that differences in these findings for p,p′-DDE and PCBs in studies of persistent organic pollutants and autism were due to differences between populations in the exposure profiles for other chemicals that interact with these pollutants. Moreover, Schmidt et al. (43) observed that folic acid intake during pregnancy attenuated the relationship between maternal insecticide exposure (determined on the basis of interviews and ecologically defined exposures to pesticides); conceivably, substances that may protect against developmental pathology from insecticides and other persistent organic pollutants differ between populations (43).

Our findings are not in agreement with those of Lyall et al. (23), who demonstrated that maternal levels of PCB 138/158 and 153 in the highest 75th percentile of the distribution were significantly associated with offspring with autism spectrum disorder. In another study of maternal levels of persistent organic pollutants associated with autism in offspring, based on a different cohort, a relatively small sample was utilized, and autistic behaviors rather than clinical diagnoses of autism spectrum disorder were included, and therefore the results may not be comparable with the results of the present study (44). The authors of that study found that maternal PCB 178 levels were associated with fewer autistic behaviors among offspring, and maternal levels of p,p′-DDE and dichlorodiphenyl trichloroethane did not show associations with the outcome.

In our study, the association between maternal levels of p,p′-DDE and autism in offspring was isolated to offspring with comorbid intellectual disability. This may suggest that the relationship between maternal exposure to p,p′-DDE and autism is related to intellectual disability in general and not to autism specifically. Previous studies have shown associations between maternal levels of p,p′-DDE and cognitive dysfunction, including reduced psychomotor development (16), general cognitive function, verbal and memory ability (17), and processing speed and verbal comprehension (18) among offspring; however, other studies have not shown these associations. For example, transplacental exposure to p,p′-DDE was associated with higher scores on the Bayley Scales of Infant Development at 6 months; the relationship disappeared at 12 months (45); and no associations were observed for maternal p,p′-DDE concentrations and scores on the Bayley Scales of Infant Development at age 8 months and on IQ at age 7 (46). If maternal exposure to DDT or p,p′-DDE has no effect on childhood neurocognition in general population samples, this may suggest that this exposure is related to a subgroup of autism cases characterized by comorbid intellectual disability, rather than to intellectual disability itself.

Although the association between maternal p,p′-DDE levels and autism was significant among male but not female offspring, the estimates of association did not differ significantly between males and females. It is possible that the lower number of female subjects hindered our ability to detect differences, if any were present, between the sexes.

Strengths and Limitations

Strengths of this study include a larger sample size than in some previous studies (22), high detection rates, and a national population-based sample. Our study had several limitations as well. First, we did not examine a comparison group of individuals with intellectual disability but without autism; hence, we cannot rule out the possibility that our finding of an association between maternal p,p′-DDE exposure and autism in offspring was accounted for by intellectual disability. However, in addition to the preceding discussion on maternal levels of p,p′-DDE and neurocognition in offspring, we note that Lyall et al. (23) reported similar associations in subgroup analyses of autism with and without comorbid intellectual disability. In their analysis of intellectual disability without autism spectrum disorder, they found a numerically increased risk for intellectual disability among offspring in the second and fourth quartiles of maternal levels of PCB 138/158 and in the third quartile of maternal levels of p,p′-DDE, although these associations were not statistically significant. Second, although the majority of mothers of case subjects in our birth cohort had serum samples tested, a significant proportion were not included. The included case subjects were born after 1993 (p<0.0001) and were more likely to be positive for a maternal (p=0.01) or parental (p=0.04) history of psychiatric disorders than those who were not included (for further details, see Table S2 in the online supplement). However, this should not have biased our results given that we accounted for these characteristics in the design and analyses. Third, we presented fresh weight serum concentrations of persistent organic pollutants, rather than lipid-adjusted concentrations. However, the unadjusted measures were found to have a low degree of bias under a range of causal scenarios (47) and to be highly correlated with lipid-adjusted concentrations (r=0.95) (26). Nonetheless, we cannot entirely rule out bias due to uncontrolled confounding by serum lipids. Fourth, we did not adjust for multiple comparisons; if we had, the association of maternal levels of p,p′-DDE with autism would have narrowly missed the Bonferroni-corrected traditional threshold for statistical significance (alpha=0.025), given that two persistent organic pollutants were tested. However, the Bonferroni method focuses on situations in which multiple statistical tests are conducted without a priori hypotheses or when testing for whether all null hypotheses are true simultaneously (48). These situations did not apply to our primary statistical tests, which were restricted to a priori hypotheses, and these were evaluated separately. Finally, although potential confounders were adjusted in the analyses, there is always the possibility, as in any observational study, of residual confounding. However, given the selectivity of our finding for maternal levels of p,p′-DDE, this does not appear to be likely, unless the potential for residual confounding is greater for p,p′-DDE compared with PCBs.

Conclusions

In this study, we demonstrated an association between maternal levels of p,p′-DDE and odds of autism in offspring. To our knowledge, this is the first study to report this relationship. There was no association between maternal exposure to PCBs and autism. Further research is necessary to replicate this finding and evaluate a potential role for maternal PCBs in autism. This study has potential implications for the prevention of autism and may provide a better understanding of its pathogenesis.