Second-trimester placental changes associated with elevated maternal serum alpha-fetoprotein.

Measuring maternal serum alpha-fetoprotein (AFP) levels in the second trimester is an effective screening test for identifying pregnancies at increased risk for neural tube defects. In the absence of a neural tube defect there are many nonpathologic and pathologic causes for elevated AFP including underestimated gestational age, twin gestation, impending fetal death, and rare fetal malformations. In this series, intraplacental sonolucent spaces were detected in a significant percentage of second trimester pregnancies with elevated serum AFP in the absence of any other cause for the elevation. It is postulated that these cystic spaces are a conduit for the transfer of fetal blood into the maternal circulation, thus accounting for the nonpathologic AFP elevation in the maternal serum.