We conducted a metaanalysis of published evidence on the relationship between maternal obesity and the risk of neural tube defects (NTDs). Eligible studies were identified from 3 sources: (1) PubMed search of articles that were published from January 1980 through January 2007, (2) reference lists of publications that were selected from the PubMed search, and (3) reference lists of review articles on obesity and maternal outcomes that were published from January 2000 through January 2007. Twelve studies met inclusion criteria. A Bayesian random effects model was used for the metaanalysis and metaregression. Unadjusted odds ratios for an NTD-affected pregnancy were 1.22 (95% CI, 0.99-1.49), 1.70 (95% CI, 1.34-2.15), and 3.11 (95% CI, 1.75-5.46) among overweight, obese, and severely obese women, respectively, compared with normal-weight women. None of the study characteristics included in the metaregression analysis affected the results significantly. Maternal obesity is associated with an increased risk of an NTD-affected pregnancy.