This study provides data on the incidence of fetal trisomies 21, 18, and 13 at 9-14 weeks' gestation in women aged 35-45 years and estimates of maternal age-specific risks in women aged 20-45 years. Our data from 5814 singleton pregnancies undergoing first-trimester karyotyping for the sole indication of maternal age > or = 35 years were combined with those from two previous reports and the incidence of the trisomies was calculated from a total of 15,793 pregnancies. Comparison of incidences at 9-14 weeks' gestation with published data at 15-20 weeks' gestation and in livebirths demonstrated that at birth the maternal age-specific incidence of trisomy 21 is 33 per cent lower than at 15-20 weeks' gestation and 54 per cent lower than at 9-14 weeks' gestation. Furthermore, the relative frequency of trisomies 18 and 13 decreases from 30 per cent at 9-14 weeks to 22 per cent at 15-20 weeks and 14 per cent at birth.