Alpha fetoprotein in maternal serum in pregnancies with trisomy 18

Maternal serum alpha-fetoprotein (MS-AFP) was assayed before amniocentesis in 2441 pregnancies of women age 35 or over and of women with prior birth of a child with a chromosome anomaly. Ten cases of trisomy 18 and 25 cases of trisomy 21 were detected. In trisomy 18 without neural tube defect we found MS-AFP levels significantly lower than in trisomy 21.