Rauch et al. newborns at risk for adverse health outcomes (Wise et al. 1985). Racial differences in ges- tational age and birth weight may be a result of adverse environmental exposures, genetics, socioeconomic status, and/or other unidenti- fied factors. Several studies in the United States have found that black individuals are often exposed to higher levels of environmental toxi- cants than white individuals, but it is unclear whether these exposures contribute to racial differences in gestational age or birth weight (Gee and Payne-Sturges 2004). There is also evidence that PON1 distributions differ across racial groups (Davis et al. 2009). The purpose of this study was to exam- ine the relationship of prenatal OP insecticide exposure, measured using urinary concentra- tions of six DAP metabolites, with gestational age and birth weight in a birth cohort from Cincinnati, Ohio. We hypothesized that ges- tational OP exposure would be associated with shorter gestation and lower birth weight. We also hypothesized that the relationship between urinary OP exposure and birth weight/ gestational age would be greatest among infants with PON1192QQ or PON1–108TT genotypes. Methods Study sample. We used data collected from mothers and newborns enrolled in the Health Outcomes and Measures of the Environment (HOME) Study, an ongoing prospective birth cohort. Subject recruitment and interviews for the HOME Study have been described else- where (Braun et al. 2010). Briefly, we identi- fied and contacted women attending seven prenatal clinics in the Cincinnati metropoli- tan area between March 2003 and January 2006. Eligibility criteria included being at least 18 years of age; living in a home built before 1978; ≤ 19 weeks of gestation; being HIV neg- ative; living within five surrounding counties; and not receiving thyroid or seizure medica- tions, or chemotherapy or radiation treatments. Institutional review boards of all involved research institutions, hospitals, and laboratories approved the study protocol. All mothers gave written informed consent for themselves and their children before enrolling. Of 1,263 eli- gible women, 468 enrolled in the study. Sixty- seven women dropped out of the study before delivery. The remaining 401 women gave birth to 389 live singleton babies, as well as 9 sets of twins and 3 still-born infants. Of those deliver- ing singletons, 344 women provided urine at both 16 and 26 weeks gestation; they make up our primary study sample. Measurement of pesticide exposure. Participants provided blood and spot urine samples at 16 and 26 weeks gestation, and within 24 hr of delivery. Urine was col- lected in polypropylene containers and stored at –20°C until shipment to the Centers for Disease Control and Prevention (CDC) for 1056 analysis. We measured six urinary DAPs— dimethylphosphate (DMP), dimethylthio- phosphate (DMTP), dimethyldithiophosphate (DMDTP), diethylphosphate (DEP), diethyl- thiophosphate (DETP), and diethyldithio- phosphate (DEDTP), which are common metabolites of about 75% of the OP insecti- cides used in the United States—using a mod- ification of the analytical method of Bravo et al. (2004) that employs gas chromatogra- phy-tandem mass spectrometry with isotope dilution calibration. Quality control (QC) materials, prepared at CDC from spiked pooled urine, were analyzed with standards, blanks, and study samples. The QC concentrations were evalu- ated using standard statistical probability rules (Caudill et al. 2008). The limits of detection (LODs) varied depending on the metabo- lite and ranged from 0.2 gμ/L for DMTP to 0.6 μg/L for DMP. For samples reporting nonzero concentrations below the LOD, we used the reported value. We imputed DAP concentrations that were reported as zero by choosing a random number between zero and the lowest nonzero value for that metabolite. We converted the metabolite concentra- tions from micrograms per liter to their molar concentrations (nanomoles per liter) and summed them to obtain overall concentrations of diethyl alkyl phosphates (ΣDEAPs: DEP, DETP, and DEDTP), dimethyl alkyl phos- phates (ΣDMAPs: DMP, DMTP, DMDTP), and of all six metabolites (ΣDAPs). We used creatinine as a marker of urine dilution and obtained creatinine-standardized DAP con- centrations by dividing metabolite concen- trations by creatinine concentration (Larsen 1972). To avoid confounding due to met- abolic changes that occur around the time of delivery, we confined our analysis to the 16- and 26-week results, and restricted the analysis to mothers with data from both time points. Because metabolite concentrations var- ied widely between time points, we averaged the concentrations of each metabolite across the two samples. Because the distribution of the averaged DAP concentrations was right- skewed, we log10-transformed the averages to achieve a more normal distribution. Outcome and covariate measures. We abstracted birth weight from medical records, and calculated gestational age from the mother’s self-reported date of last menstrual period. When this information was not avail- able, we used the results of an ultrasound (n = 7) or a Ballard examination performed just after delivery (n = 3) to determine gesta- tional age. Demographic factors (i.e., maternal age, education, race/ethnicity, parity, income, marital status) were obtained from subject interviews. We also characterized infant size using race-specific birth weight for gestational age z-scores (e.g., birth weight standardized for gestational age) to determine whether infants’ growth was restricted relative to their length of gestation (Oken et al. 2003). We measured serum cotinine, a metab- olite of nicotine that has been validated as a biomarker of secondhand and active tobacco smoke exposure (Benowitz et al. 2009; DeLorenze et al. 2002), and whole blood lead concentrations from blood samples taken at the same time as the spot urine samples. As with the urinary DAP concentrations, we averaged serum cotinine and blood lead concentrations across the 16- and 26-week samples and log10- transformed the results. Several women were missing lead or cotinine measurements for one or both time points; if a lead or cotinine mea- surement was missing for one time point, we used the value for the other time point instead of the average. Participants who were missing blood lead or serum cotinine concentrations from both time points (n = 18) were excluded from the analysis. Blood lead and serum coti- nine were quantified using previously described methods at the CDC laboratories (Bernert et al. 2000; CDC 2003). P O N g e n o t y p i n g d a t a . D N A w a s extracted from frozen archived cord blood using the 5PRIME PerfectPure DNA blood kit (5PRIME, Gaithersburg, MD, USA). Genotyping was conducted at the Cincinnati Children’s Hospital Medical Center DNA sequencing and genotyping core facility. We used the Applied Biosystems predesigned TaqMan assays (Applied Biosystems, Carlsbad, CA, USA) for rs662 (192Q/R) and rs705379 (–108C/T) with 15 ng of genomic DNA. The protocol was as per the manufacturer’s recom- mendations in 384 well format, including 16 blanks and 4 sets of 2 controls for QC pur- poses. Assay plates mixing reagents and DNA were prepared using a TECAN Evo200 robotic laboratory workstation (TECAN, Durham, NC, USA) and amplified in an ABI 9700 ther- mocycler (Applied Biosystems). TaqMan results were read on an ABI 7900HT real time PCR system and exported in Excel format using the ABI SDS 2.4 software (Applied Biosystems). Genotype data was available for 312 of 344 (90.7%) of infants, with 300 (87.2%) having identifiable SNPs at both PON1 sites. Statistical analysis. For multivariable anal- ysis, we used PROC GLM in SAS version 9.1 (SAS Institute Inc., Cary, NC, USA). We began with an a priori set of covariates that have been consistently associated with ges- tational age and birth weight: maternal age, education, race/ethnicity, and marital status; household income; gestational age at initia- tion of prenatal care; and parity. We also con- sidered the following covariates: depressive symptoms (measured by the Beck Depression Inventory-II) (Beck et al. 1996), IQ, insurance status, area of residence, maternal body mass index, alcohol use during pregnancy, child ovlume 120 | number 7 | July 2012 • Environmental Health Perspectives