ObjectiveTo determine if a false-positive trisomy 18 multiple-marker screening test (all three analytes low: maternal serum alpha-fetoprotein [AFP] at most 0.75 multiples of the median [MoM], unconjugated estriol at most 0.60 MoM, and hCG at most 0.55 MoM) indicates increased risk for obstetric complications or is related to maternal weight.MethodsWe accessed our genetic database to obtain multiple-marker screening test results, fetal karyotypes, and pregnancy outcomes from all patients wit a normal multiple-marker screening test (n= 3900) and from all patients with a positive trisomy 18 screening test (n= 103) seen in the prenatal diagnosis clinic from 1992 to 1996. During this period, only maternal serum AFP was adjusted for maternal weight.ResultsA positive trisomy 18 screen identified five of 12 trisomy 18 fetuses. Women with a false-positive trisomy 18 screen were heavier (175.6 ± 43.8 Ib versus 159.9 ± 37.9 Ib,P< .001) and younger (29.7 ± 6.5 years versus 32.3 ± 6.5 years,P< .001) than women with a normal multiple-marker screening test, but were not at increased risk for pregnancy complication. Weight-adjusting all three analytes reduced the false-positive trisomy 18 screen rate by 42% (from 1.9% to 1.1%) but did not change the trisomy 18 detection rate.ConclusionA false-positive trisomy 18 screening test does not indicate increased risk to develop pregnancy complications and may be related to inadequate correction for increased maternal weight.