Maternal serum alpha-fetoprotein in twin pregnancies uncomplicated by neural tube defect.

The maternal serum alpha-fetoprotein concentration was measured between 16 and 20 weeks in 145 twin pregnancies in which neither fetus had a neural tube defect. When the maternal serum alpha-fetoprotein concentration was less than two multiples of the singleton median, pregnancy outcome was good; the extended perinatal mortality rate was 32.6/1000, mean birth weights for the first and second twins were 2507 and 2443 gm, respectively, and mean gestation at delivery was 36 weeks, 6 days. When the maternal serum alpha-fetoprotein concentration was greater than four multiples of the singleton median, the outcome was poor; the extended perinatal mortality was 400/1000, mean birth weights were 1963 and 1523 gm, and mean gestation at delivery was 32 weeks, 4 days. The negative correlations of maternal serum alpha-fetoprotein concentration with birth weight and gestation at delivery were highly significant. Maternal serum alpha-fetoprotein concentration in midpregnancy is a useful predictor of outcome in twin pregnancy, independent of the occurrence of neural tube defect, and it appears to be related to the timing of delivery rather than fetal growth.