Abnormal amniotic fetal antigen 2 concentrations in trisomy 18 and trisomy 21.

Fetal antigen 2 (FA-2) is a human protein, first identified in amniotic fluid, and shown to be identical to the aminopropeptide of the alpha 1 chain of collagen type I. It exists in several different size and charge forms. In the present study, FA-2 was measured in amniotic fluid using two different assays: a rocket line immunoelectrophoretic assay which measured total FA-2, and a radioimmunoassay which was specific for the high molecular mass forms of FA-2. Both assays gave similar results. FA-2 concentrations were measured in amniotic fluid samples collected from normal pregnancies at 10-23 weeks gestation; they were shown to rise steeply from 10-14 weeks, peak at 17 weeks and then fall slightly by 23 weeks. Comparison between amniotic fluid from normal pregnancies and pregnancies affected by trisomy, showed significantly higher FA-2 concentrations in trisomy 21 and significantly lower concentrations in trisomy 18.