The goal of this study was to determine the prevalence and the nature of congenital anomalies found at birth in offspring of women who had a normal second-trimester ultrasound and/or amniocentesis. Two groups of women were studied in our prenatal diagnosis clinic between 1991-1997. Group 1 consisted of pregnant women who had an amniocentesis for advanced maternal age (AMA), or for familial chromosomal or monogenic disorders. Group 2 consisted of pregnant women attending the prenatal diagnosis clinic and who had no indication for amniocentesis. Those with an abnormal ultrasound and/or amniocentesis were excluded. At the time of delivery, a questionnaire was sent pertaining to perinatal complications and the anomalies detected during the neonatal period. From a total of 15, 370 questionnaires sent from 1991-1997, 10,823 (group 1, n = 8,877; group 2, n = 1,946) were returned (overall response rate, 70.4%). Mean maternal age was 36 years in group 1 and 29 years in group 2. The prevalence of perinatal complications was similar in the two groups. In each group, the prevalence of all unforeseen anomalies was 2.9%. In group 1, the distribution of those anomalies was: major anomalies, 67.7%; minor anomalies, 23.9%; and multiple congenital anomalies (MCA), 8.3%. In group 2, the distribution was: major anomalies, 70.7%; minor anomalies, 24.1%; and MCA, 5.2%. In patients at risk for a genetic disease and consulting in a prenatal diagnosis clinic, the prevalence of all anomalies diagnosed at birth was 2.9%, even if the second-trimester ultrasound and amniocentesis results were normal. Therefore, it is important to inform those couples of this remaining risk.