Clinical implications of amniotic fluid alpha-fetoprotein in twin pregnancy.

Amniotic fluid alpha-fetoprotein (AF-AFP) was measured in 68 pairs of normal twins. Acetylcholinesterase (ACHE) was measured in 23 of those pairs. A significant difference in AF-AFP levels was found in 9/49 (19.4%) of twin pairs of the same sex and 9/19 (47.3%) of twin pairs of opposite sexes (P = .015). Differences in fetal size or gestational age at amniocentesis did not influence the rate of discordant AFP results. ACHE activity was identical in all amniotic fluid samples in twin pairs. In five additional patients, fetal anomalies (three), fetal death or fetal blood in the sample could have affected AFP levels, which were found to be discordant in three of them. Using fetal sex to represent zygosity, the study indicated that discordance in AF-AFP is more common in dizygous twins. The odds ratio for having a discordant AFP result if twins are of different sexes was 4.0. Implications of our data are: (1) in same-sex twins, discordance between sacs is less common, even if only one fetus is affected; (2) the difference in AFP results in twins of the same sex and of different sexes suggests variability in AFP transfer between twin gestational sacs; and (3) ACHE readily diffuses between sacs and cannot be used to determine which twin has an abnormality.