We studied 253 women with a pregnancy complicated by a birth defect and 506 controls to determine the frequency and type of prenatal tests and the types of defects detected antenatally. Most women had at least one ultrasound examination, but the frequency of other screening tests was low. Only 38 (15%) of defects were detected antenatally (37 by ultrasound). Birth prevalence is unlikely to be affected by pregnancy termination.