Picture Vocabulary test), Home Observation for Measurement of the Environment (HOME) score, season of urine collection, maternal PON1 activity or genotype, lan- guage spoken in the home, and exact age at testing. Gestational age at delivery and birth weight were not evaluated for confounding because they are potentially causal intermedi- ates (Moreno-Banda et al. 2009; Whyatt et al. 2004; Wolff et al. 2007). All models were adjusted for examiner and urinary creatinine. Results The Mount Sinai Children’s Environmental Health Center enrolled a multiethnic inner- city cohort, most of whom were black or Hispanic women (~ 80%) (Table 1). Most mothers in the cohort were < 25 years of age at enrollment. However, in some follow-up years, those who returned for follow-up assess- ments tended to be older women, with a dis- proportionately low fraction of women in the youngest category (< 20 years at enroll- ment) being unreachable by any of our con- tact methods. We found similar trends for education. Those returning for follow-up did not differ substantially from the originally enrolled cohort with respect to racial/ethnic Original enrolled cohort (n = 404) 12 months (n = 200) Engel et al. included an indicator variable to account for whether the scores derived from a WISC-IV or WPPSI-III exam. The convergent validity between the WISC-IV and WPPSI-III has been previously reported. Specifically, in psy- chometric analyses, the correlation between WPPSI-III and WISC-IV FSIQ scores was 0.89; WPPSI-III Verbal IQ and WISC-IV Verbal Comprehension, 0.83; and WPPSI-III Performance IQ and WISC-IV Perceptual Reasoning, 0.79 (Flanagan and Kaufman 2004). In our study, among children who returned for both exams (n = 103), the cor- relations between the composite scores on the WPPSI-III and WISC-IV were likewise very strong: FSIQ = 0.83, Verbal IQ/Verbal Comprehension = 0.84, and Performance IQ/ Perceptual Reasoning = 0.78. For all mod- els, covariates were retained if their exclu- sion caused more than a 10% change in the β-coefficient of the full model or if they improved the precision of the main effect estimate. The following covariates were con- sidered as potential confounders or effect modifiers: maternal age, race/ethnicity, marital status, education, breast-feeding, child sex, alcohol, smoking, or drug use during preg- nancy, maternal IQ (measured by the Peabody Center, 1998–2002 [n (%)]. Characteristic Maternal age at delivery (years) Race/ethnicity < 20 20–24 25–29 30–34 ≥ 35 White Black Hispanic Other Married Single Education Marital status Living with baby’s father < High school High school graduate Some college ≥ Bachelor’s degree Alcohol use during pregnancy Duration of breast-feeding (months) < 1 1–3 ≥ 4 Any organophosphate biomarker level Maternal PON1 Q192R QQ QR RR Maternal paraoxonase enzyme activity (units/mL) 2,964–9,576 (tertile 1) 9,700–11,660 (tertile 2) 11,665–20,000 (tertile 3) 1184 Table 1. Characteristics of the Mount Sinai Children’s Environmental Health Study, Mount Sinai Medical composition. Importantly, we also found no meaningful differences with respect to breast- feeding behaviors or alcohol use during preg- nancy. In general, mothers who returned for follow-up assessments tended to have been older at enrollment and to have achieved a higher level of education. Allele frequencies for PON1 polymorphisms varied by race, as has been previously reported for our population (Chen et al. 2003). In the population of sub- jects included in this analysis, the frequency of the A allele (resulting in Q amino acid) was 74% among whites, 36% among blacks, and 55% among Hispanics. The frequencies of the –108T allele and 55L alleles were also differen- tial by race. For a detailed description of allele frequencies and metabolite concentrations according to race, see Supplemental Material, Table 1 (doi:10.1289/ehp.1003183). At the 12-month BSID-II exam, the esti- mated effect of organophosphate metabolites on the MDI was strongly heterogeneous by race/ethnicity for the ΣDAP and ΣDMP metabolites (Table 2). Among nonwhites, increasing ΣDAP and ΣDMP tertiles of expo- sure were associated with a decrease in the MDI [log10 ΣDAP: β = –3.29; 95% confi- dence interval (CI), –5.88 to –0.70]. However, among whites, the reverse pattern emerged, with higher exposure associating with better MDI scores (log10 ΣDAP: β = 4.77; 95% CI, 0.69–8.86). We found similar trends in the effect estimates when we stratified by housing type (public vs. private) rather than race/eth- nicity. We found no heterogeneity in ΣDEP effect estimates according to race/ethnicity, and overall, ΣDEP metabolites were not asso- ciated with the 12-month BSID-II MDI. We found no relationship between organophos- phate metabolites and the PDI at 12 months overall, and no interaction with race/ethnicity for any of the metabolite groups (Table 2). At the 24-month BSID-II, effect estimates were not heterogeneous by race/ethnicity (data not shown). Consistent with the 12-month assessment, prenatal maternal ΣDAP metabo- lite level was inversely associated with the 24-month MDI (β = –2.08; 95% CI, –4.60 to 0.44) in multivariate adjusted models, although the effect estimates were attenu- ated relative to the 12-month estimates and measured with comparable precision [see Supplemental Material, Table 2 (doi:10.1289/ ehp.1003183)]. The metabolites were not associated with 24-month PDI. Because of the strong interaction between race/ethnicity and organophosphate metabo- lites on the BSID-II MDI at 12 months, we examined the interaction between the PON1 polymorphisms and ΣDAP and ΣDMP metabolites within strata of race/ethnicity; however, the white population was too small to further subdivide by genotype. Therefore, we restricted this analysis to only the black ovlume 119 | number 8 | August 2011 • Environmental Health Perspectives Follow-up 24 months (n = 276) 96 (34.8) 83 (30.1) 34 (12.3) 43 (15.6) 20 (7.2) 63 (22.8) 74 (26.8) 136 (49.3) 3 (1.1) 81 (29.4) 62 (22.5) 133 (48.2) 78 (28.3) 55 (19.9) 70 (25.4) 73 (26.5) 36 (13.4) 122 (44.2) 56 (20.3) 98 (35.5) 267 (96.7) 78 (30.2) 119 (46.1) 61 (23.6) 91 (35.7) 81 (31.8) 83 (32.6) 6–9 years (n = 169) 54 (31.9) 56 (33.1) 25 (14.8) 19 (11.2) 15 (8.9) 31 (18.3) 47 (27.8) 88 (52.1) 3 (1.8) 38 (22.5) 40 (23.7) 91 (53.9) 46 (27.2) 36 (21.3) 51 (30.2) 36 (21.3) 28 (16.9) 73 (43.5) 36 (21.4) 59 (35.1) 165 (99.0) 46 (28.2) 78 (47.9) 39 (23.9) 57 (34.8) 43 (26.2) 64 (39.0) 142 (35.2) 132 (32.7) 44 (10.9) 64 (15.8) 22 (5.4) 86 (21.29) 112 (27.72) 200 (49.50) 6 (1.49) 117 (29.0) 98 (24.3) 189 (46.8) 118 (29.4) 83 (20.7) 103 (25.6) 98 (24.4) 59 (14.0) 140 (42.2) 71 (21.4) 121 (36.5) 383 (94.8) 120 (30.9) 174 (44.7) 95 (24.4) 130 (33.9) 123 (32.1) 130 (33.9) 57 (28.5) 56 (28.0) 30 (15.0) 39 (19.5) 18 (9.0) 57 (28.5) 52 (26.0) 89 (44.5) 2 (1.0) 71 (35.5) 41 (20.5) 88 (44.0) 50 (25.0) 35 (17.5) 49 (24.5) 66 (33.0) 31 (16.0) 74 (37.0) 45 (22.5) 81 (40.5) 190 (95.0) 63 (33.3) 83 (43.9) 43 (22.8) 69 (37.5) 51 (27.7) 64 (34.8)