Background: Approximately 35 percent of all cases of Down's syndrome in fetuses can be detected by measuring maternal serum alpha-fetoprotein during the second trimester in the general population of pregnant women. Recent case-control studies indicate that this detection rate could be approximately doubled by measuring serum levels of unconjugated estriol and chorionic gonadotropin, which are abnormally low and abnormally high, respectively, in women carrying fetuses affected by Down's syndrome.
Methods: We prospectively screened 25,207 women and adolescents in the second trimester of pregnancy and assigned each a risk of fetal Down's syndrome with an algorithm that took into account measurements of all three serum markers in combination with maternal age. On this basis, 1661 subjects (6.6 percent) were initially assigned a second-trimester risk of fetal Down's syndrome of at least 1 in 190, and 962 (3.8 percent) were offered amniocentesis for chromosomal analysis after verification of gestational age. Gestational age was determined on the basis of the first day of the last menstrual period or, when available, by ultrasonography.
Results: Among the 760 women and adolescents who chose amniocentesis, 20 cases of fetal Down's syndrome were detected, along with 7 other chromosomal disorders. There was 1 additional case of fetal Down's syndrome among the 202 women who chose not to have amniocentesis. The rate of detection of Down's syndrome was thus 58 percent (21 of 36 expected cases), and the frequency of identifying a fetus with Down's syndrome in women undergoing amniocentesis was 1 per 38 amniocenteses (95 percent confidence interval, 1 in 25 to 1 in 62).
Conclusions: Measuring serum alpha-fetoprotein, chorionic gonadotropin, and estriol is more effective in screening for fetal Down's syndrome than measuring maternal serum alpha-fetoprotein alone. Such an expanded protocol can readily be incorporated into existing prenatal screening programs.