Screening for aneuploidy in twin pregnancies: Maternal age- and race-specific risk assessment between 9-14 weeks

The aim of this study was to calculate the risk for aneuploidy in twin pregnancies between 9-14 weeks utilizing maternal age, race and dizygotic twinning rates. Using previously published risks for aneuploidy in singletons and twins at the time of amniocentesis and at term, we calculated new risk estimates for twins at 9-14 weeks gestation or at the time of chorionic villus sampling. Using these tables, the risk for trisomy 21 in at least one fetus of a twin gestation in a 32-year-old at 9-14 weeks is 1/285 for Whites and for African-Americans. This is equivalent to the risk for trisomy 21 (1/265) in a 35-year-old woman with a singleton at the same gestational age. The risks for trisomies 18 and 13 also follow similar trends. In counseling women with twin pregnancies at the time of first trimester nuctranslucency screening or chorionic villus sampling, it should be noted that the maternal age-related risk for aneuploidy for a 32-year-old is equivalent to that of a 35-year-old woman with a singleton gestation, combined with biochemical markers (Snijders et al., 1998; Spencer et al., 1999) has been established in several countries and preliminary studies in twin gestations have reported detection rates for trisomy 21 between 75-88% (Sebire et al., 1996; Spencer et al., 2000). Our aim is to calculate the risk of aneuploidy in twin pregnancies between 9-14 weeks gestation, taking into account the maternal age, gestational age and the influence of race on dizygotic twinning rate. We believe that it is important to have the most accurate numerical risks available to include in the counseling of women considering NT screening or prenatal diagnosis using CVS.