Objective:Interpregnancy interval (IPI) influences numerous adverse perinatal outcomes. IPI's impact on birth defects is unclear.Study Design:We conducted a population-based case-control study, using 1998 to 2008 administrative data from Washington State. A total of 10 772 cases, women whose second of two births resulted in an infant with a birth defect, were compared with 32 310 controls, women whose second of two births did not result in an infant with a birth defect. Result:Compared with mothers with an IPI between 18 to 23 months, those with an IPI <6 months or ≥60 months had elevated risks of delivering an infant with a birth defect (odds ratio=1.15, 95% confidence interval: 1.03 to 1.28, and odds ratio=1.15, 95% confidence interval: 1.04 to 1.26, respectively).Conclusion:We observed a J-shaped relationship between IPI and risk of having an infant with a birth defect. As this is one of the first studies to evaluate this association, confirmatory studies are needed. © 2012 Nature America, Inc. All rights reserved.