First-trimester risk assessment for trisomies 21 and 18 in twin pregnancy.

Abstract

Our objective was to describe performance of first-trimester combined risk assessment in twin pregnancies. Twin pregnancies that underwent risk assessment in our ultrasound unit from 2003-2006 were included. Adjusted risks for trisomies 21 and 18 that were based on age, nuchal translucency (NT), and biochemistry were provided for each twin. Detection rates for Down syndrome and trisomy 18 were calculated for age/NT, and age/NT/biochemistry at a screen-positive rate of 5% of pregnancies. Five hundred thirty-five pregnancies were included. Median maternal age was 34 years, with 47% of women > or = 35 years old. There were 7 fetuses in 6 dichorionic pregnancies with Down syndrome and 3 fetuses in 3 pregnancies with trisomy 18. For a 5% false-positive rate, age/NT identified 83.3% of Down syndrome and 66.7% of Trisomy 18 pregnancies. Adding biochemistry resulted in 100% detection rates for both conditions. The addition of biochemistry may enhance first-trimester risk assessment in twin pregnancies. Further studies with larger numbers of affected pregnancies are needed.