Maternal serum markers for fetal trisomy 21 screening

A population at increased risk for fetal trisomy 21 can be defined by means of maternal serum markers. Various markers have been used since 1984, and the following have proved most valuable: hCG, free beta hCG, AFP, and estriol. Two prenatal screening periods should be distinguished: first trimester (8-14 weeks) and second trimester (14-18 weeks). Only the latter has been prospectively evaluated. In a prospective study, we assayed hCG in second trimester serum. A risk factor combining maternal age and hCG was defined and amniocentesis was offered to patients at increased risk for fetal trisomy 21. Out of 51,048 patients under 38 years of age, 135 had a trisomy 21-affected fetus. In 36,697 patients under 35, we observed 70 cases of trisomy 21, of which 41 (59%) were in the group at risk. Karyotyping was performed in 7.1% of these patients. In 11,351 patients aged 35-37 years, there were 65 cases of trisomy 21, of which 52 (80%) were in the group at risk. Karyotyping was performed in 26.8% of these patients. In our experience, parallel assaying of maternal serum AFP only detects a further 1% of trisomy 21 pregnancies for the same number of amniocenteses. These results confirm the findings of all previous prospective studies: maternal hCG screening is the most effective method of detecting trisomy 21 in the general population.