Estimates are made of the rates of all clinically significant cytogenetic abnormalities in live births by 1-year maternal age intervals ranging from ages 15 to 49. Down syndrome results from the most prevalent clinically significant cytogenetic abnormality, yet the rates of disorders associated with other cytogenetic abnormalities also increase with maternal age and significantly contribute to the overall rate at the older ages. The estimated rate of all clinically significant cytogenetic abnormalities rises from about 2 per 1000 (1 per 500) at the youngest maternal ages to about 2.6 per 1000 (1 per 270) at age 30, 5.6 per 1000 (1 per 180) at age 35, 15.8 per 1000 (1 per 60) at age 40, and 53.7 per 1000 (1 per 20) at age 45.