Maternal age- and gestation-specific risk for trisomy 21

Objective To provide estimates of maternal age- and gestational age-related risks for trisomy 21. Methods The prevalence of trisomy 21 was examined in 57 614 women who had f etal karyotyping at 9-16 weeks of gestation for the sole indication of mate rnal age of 35 years or more. On the basis of the maternal age distribution and the reported maternal age-related risk for trisomy 21 at birth, the ex pected number of trisomy 21 cases was calculated for each gestational age s ubgroup (9-10 weeks, 11-14 weeks and 15-16 weeks). The ratio of the observe d to expected number of cases of trisomy 21 was then calculated and regress ion analysis was applied to derive a smoothened curve. The formula for mate rnal age- and gestational age-related risk was then applied to a population of 96 127 pregnancies that were examined at 10-14 weeks to calculate the e xpected number of trisomy 21 pregnancies, and this number was compared to t he observed number of 326. Results In the 57 614 pregnancies there were 538 cases of trisomy 21. The r elative prevalences of trisomy 21, compared to a prevalence of 1.0 at 40 we eks, was 10 exp(0.2718 x log(10)(gestation)(2) - 1.023 x log(10)(gestation) + 0.9425). On the basis of the estimated maternal age- and gestational age -related risks, the expected number of trisomy 21 cases at 10-14 weeks of g estation in the 96 127 pregnancies was 329 (95% confidence interval 291-361 ), which was not significantly different from the observed number of 326 ca ses (chi(2) = 0.02). Conclusion The risk for trisomy 21 increases with maternal age and decrease s with gestation. The prevalence of trisomy 21 at 12 and 16 weeks of gestat ion is higher than the prevalence at 40 weeks by 30% and 21%, respectively.