OBJECTIVE: To estimate the maternal age-specific live birth prevalence (in the absence of prenatal diagnosis and selective termination) of trisomy 13 (Patau syndrome) and trisomy 18 (Edwards syndrome) and compare it with that of trisomy 21 (Down syndrome). METHODS: Records of prenatal and postnatal diagnoses from seven UK regional congenital anomaly registers and two Australian registers covering 4.5 million births included 975 diagnoses of trisomy 13 and 2254 of trisomy 18. Prevalence at birth in the absence of prenatal diagnosis and selective termination was calculated by adjusting for prenatally diagnosed pregnancies that were terminated according to their likelihood of surviving to term. RESULTS: The live birth prevalence in the absence of prenatal screening and selective termination in England and Wales from 1997 to 2004 was 1.4 (95% CI: 1.2-1.6) per 10 000 births for trisomy 13 and 2.3 (95% CI: 2.1-2.5) for trisomy 18. It has increased since 1989-1996, by 13% for trisomy 13 and 25% for trisomy 18. These increases are consistent with those predicted due to increases in maternal age. CONCLUSION: This study provides the first estimates of maternal age-specific prevalence of trisomies 13 and 18 for women aged 16-45.