Nonsyndromic atrioventricular septal defects (AVSDs) are serious congenital heart defects for which information on prevalence and descriptive characteristics based on large, geographically, and ethnically diverse populations has been limited. To describe the birth prevalence and phenotype of nonsyndromic AVSDs, we used data from the National Birth Defects Prevention Study (NBDPS), a multisite, population-based case–control study aimed at identifying genetic and environmental risk factors for birth defects. For this analysis, infants born during the period 1997–2005 and meeting the NBDPS case definition for AVSDs were included. Infants with an AVSD associated with recognized or strongly suspected chromosomal abnormalities or single-gene disorders (syndromic case infants) were excluded. We identified 302 infants with a nonsyndromic AVSD for a birth prevalence of 0.83/10,000 livebirths. Over 20% of infants with an AVSD had an additional major birth defect, with gastrointestinal, renal or urinary, and central nervous system defects being the most common. A lower prevalence of AVSDs was seen among infants born to Hispanic mothers compared with those born to non-Hispanic White mothers [prevalence ratio = 0.63 (95% confidence interval: 0.46–0.86)]. Understanding the prevalence of nonsyndromic AVSDs, demographic factors associated with their occurrence, and associated defects could help guide clinical care, as well as contribute to a better understanding of pathogenesis. © 2011 Wiley-Liss, Inc.