Objective.-To investigate the risk for neural tube defect (NTD)-affected pregnancies among obese women (ie, women with a body mass index [BMI] >29 kg/m(2)) compared with women of average prepregnancy weight. Design.-Population-based case-control study. Setting.-All hospitals in 55 of 58 counties in California. Participants.-In-person interviews were conducted with mothers of 538 (88% of eligible) NTD cases (including fetuses and infants electively terminated, stillborn, or born alive) and with mothers of 539 nonmalformed controls (88%) within an average of 5 months from the term delivery date. Main Outcome Measures.-The risk of an NTD-affected pregnancy among obese women. Results.-Compared with women whose BMI was less than or equal to 29 kg/m(2), an increased risk for NTD-affected pregnancy was observed among obese women (odds ratio, 1.9; 95% confidence interval, 1.3 to 2.9). The increased risk was not attributable to maternal nonuse of a vitamin containing folic acid, diabetes, use of diet pills, lower dietary folate intake, or an NTD-pregnancy history. Adjustment for maternal age, education, gravidity, use of vitamins, and use of alcohol did not change the odds ratio. The risk associated with maternal obesity was greater for spina bifida and for other less prevalent NTDs than for anencephaly. Conclusion.-Because as many as 10% of women maybe obese periconceptionally, the observed twofold increased risk is relevant to the population burden of NTDs.
