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

OBJECTIVE: Our objective was to describe performance of first- trimester combined risk assessment in twin pregnancies. STUDY DESIGN: 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. RESULTS: Five hundred thirty- five pregnancies were included. Median maternal age was 34 years, with 47% of women >= 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. CONCLUSION: The addition of biochemistry may enhance first- trimester risk assessment in twin pregnancies. Further studies with larger numbers of affected pregnancies are needed.