RISK-BASED PRENATAL SCREENING FOR TRISOMY-18 USING ALPHA-FETOPROTEIN,UNCONJUGATED ESTRIOL AND HUMAN CHORIONIC-GONADOTROPIN

Nine centres collaborated to examine the feasibility of a screening me thod for trisomy 18 that was based on assigning individual risk, using a combination of maternal age and measurements of alpha-fetoprotein ( AFP), unconjugated oestriol (uE3), and human chorionic gonadotropin (h CG). Second-trimester measurements of these analytes were obtained fro m 94 trisomy 18 pregnancies. In the 89 pregnancies without an associat ed open defect, the median levels for AFP, uE3, and hCG were 0.65, 0.4 3 and 0.36 multiples of the unaffected population median, respectively . The strongest individual predictor of risk for trisomy 18 was uE3, f ollowed by hCG, AFP, and maternal age, in that order. Using a method o f individual risk estimation that is based on the three markers and ma ternal age, 60 per cent of pregnancies associated with trisomy 18 woul d be detected at a risk cut-off level of 1:100, with a false-positive rate of about 0.2 per cent. One in nine pregnancies identified as bein g at increased risk for trisomy 18 would be expected to have an affect ed pregnancy. This risk-based screening method is more efficient than an existing method that is based on fixed analyte cut-off levels. Even though the birth prevalence of trisomy 18 is low, prenatal screening can be justified when performed in conjunction with Down syndrome scre ening and when a high proportion of women offered amniocentesis have a n affected fetus.