Background: The characteristics and methodologies of state-based birth defect surveillance systems might influence reported prevalence rates, making comparisons among states difficult. Standardizing methods to minimize variability beyond true differences in prevalence will aid national surveillance efforts and birth defects prevention programs. Methods: Using data provided in the January 2000 Congenital Malformations Surveillance Report from the National Birth Defects Prevention Network, we characterized the surveillance methodologies among all sites. We then identified prevalence rates that are highly varied among systems that use each of our specified methodologies. We also examined the standards used by other collective health registries that exist across geographical boundaries. Results: Large differences in prevalence rates across case ascertainment methods (active, passive, or combination of both) were observed for some conditions, but not for others. We identified additional factors which may influence prevalence rates, including case ascertainment sources, case inclusion criteria, and inclusion of elective terminations and stillbirths. The impact of each of these factors on prevalence rates may be defect-specific. Conclusions: We conclude that while some variability is expected due to differences in the true prevalence of birth defects, extreme differences among states are more likely due to differences in surveillance practices. The Birth Defects Prevention Act prompted new initiatives to develop birth defect surveillance systems, but there are no nationally agreed upon standards in existence to guide the process. This study was performed in support of developing standards that will influence new and existing state surveillance systems. Teratology 64:S8-S13, 2001. (C) 2001 Wiley-Liss, Inc.
