Amniotic fluid alpha-fetoprotein levels and fetal chromosomal abnormalities.

406 midtrimester amniotic fluid samples were examined for alpha-fetoprotein (AFP) levels and fetal karyotyping. 44 cases with Down syndrome, 12 with Klinefelter syndrome and 14 with other chromosomal abnormalities were diagnosed. The AFP levels were below the median value for the normal pregnancies in 38 of the 44 Down syndrome-affected fetuses. The mean AFP value was significantly lower (p less than 0.0001) in this group. No such difference was found for fetuses affected by Klinefelter syndrome or other chromosomal aberrations. Almost all of the values for the Down syndrome-affected fetuses were, however, within the normal range, thereby precluding the possibility of using this measurement as an alternative to fetal karyotyping as a screening test for Down syndrome.