The anatomic basis of maternal serum screening.

Fetal serum markers, such as alpha fetoprotein (AFP), must traverse one of two very different pathways to reach maternal serum, either from fetus to amnion fluid, membranes and decidua or from fetal to maternal circulation through the placental villi. Alpha fetoprotein usually enters the amnion fluid through body wall defects uncovered by skin or through urine. Placental AFP leakage may be from villous hemorrhage or injury. These observations from anatomic pathology suggest that biochemical markers may exist to identify the source of elevated maternal serum AFP.