BACKGROUND: Birth defects affect 3% of babies born, and are one of the leading causes of infant mortality. Both younger and older maternal age may pose increased risks for certain birth defects. This study assessed the relationship between maternal age at the estimated delivery date and the risk for birth defects. METHODS: Data were obtained from the National Birth Defects Prevention Study, a population-based case-control study including mothers across 10 states. Maternal age was stratified into six categories: <20, 20 to 24, 25 to 29, 30 to 34, 35 to 39, and ≥40 years, and also analyzed as a continuous variable. Logistic regression models adjusted formaternal race/ethnicity, education, body mass index (BMI), folic acid use, smoking, gravidity, and parental age difference were used to estimate adjusted odds ratios (aORs) and 95% confidence intervals (CIs). RESULTS: For maternal age <20 years, associations with total anomalous pulmonary venous return (aOR, 2.3; 95% CI, 1.3–4.0), amniotic band sequence (aOR, 2.4; 95% CI, 1.5–3.8), and gastroschisis (aOR, 6.1; 95% CI, 4.8–8.0) were observed. For the ≥40 year age group, associations with several cardiac defects, esophageal atresia (aOR, 2.9; 95% CI, 1.7–4.9), hypospadias (aOR, 2.0; 95% CI, 1.4–3.0), and craniosynostosis (aOR, 1.6; 95% CI, 1.1–2.4) were observed. Results using maternal age as a continuous variable were consistent with those that used categorized maternal age. CONCLUSION: Elucidating risk factors specific to women ateither extreme of maternal age may offer prevention opportunities. All women should be made aware of prevention opportunities, such as folic acid supplementation, to reduce the occurrence of birth defects. Birth Defects Research (Part A), 2012. © 2012 Wiley Periodicals, Inc.