Birth Defects

Abstract Major structural birth defects occur in about 3% of live births. Methodological problems with the study of birth defects and their causes start with measuring their risk. Incidence is the usual epidemiologic measure of disease occurrence, but incidence at the time of fetal development is not measurable for birth defects—only prevalence at birth. The prevalence of most birth defects has remained fairly steady over time. There are well-known examples of medications and infections that increase the prevalence of specific birth defects. There are also concerns about environmental toxicants that might damage the fetus. Even so, most birth defects have no known cause, and most may be the inevitable product of complex embryo assembly, modified by genetic variations that confer vulnerability to specific types of defects.