Second-trimester Down syndrome maternal serum screening in twin pregnancies: impact of chorionicity

Objective To evaluate the diagnostic value of second-trimester maternal serum screening for Down syndrome in twin pregnancies. Method On the basis of a prospective study of second-trimester maternal serum screening, we studied the distribution of alpha-fetoprotein (AFP) and free B hCG in 3043 twin pregnancies with known outcome. There were 1561 dichorionic and 244 monochorionic pregnancies. The placental type was not available in 1238 cases. We compared 5 screening policies with the same risk, 1/250, cut-off: maternal age, maternal age corrected for the risk of having at least one affected twin in dichorionic pregnancies, maternal serum marker screening using observed AFP and free B-hCG values divided by a factor of 2, by using the median values actually observed in the global twin population, or by the median values specific to mono- or dichorionic twins. Results When expressed in singleton-derived MoMs, the median was 2.10 for AFP and 2.11 for free B-hCG. The median AFP did not differ between monochorionic and dichorionic pregnancies. The distribution of free B-hCG was significantly shifted towards greater values in monochorionic (2.16 MoM) compared to dichorionic (2.07) pregnancies (p < 0.0001). Screened-positive and detection rates were, respectively, 6.6% and 27.3% using maternal age alone, 24.6% and 54.5% using maternal age corrected for the risk of having at least one affected twin in dichorionic pregnancies, 7.75% and 54.5% using observed AFP and free beta-hCG values divided by a factor of 2, 8.05% and 54.5% using the median values actually observed in the global twin population, and 7.75% and 54.5% using the median values specific to mono- or dichorionic twins. Conclusion Trisomy 21 second-trimester maternal serum screening is feasible in twins, and is better than a policy based on maternal age alone. Copyright (C) 2003 John Wiley Sons, Ltd.