Relationship between human serum placental alkaline phosphatase and the ABo and Rh compatibility status in pregnancy.

458 unrelated healthy women at various gestational ages were examined for serum heat-stable alkaline phosphatase (HSALP) activity. The sample was subdivided into four groups according to the compatibility mating type in the ABO and Rh systems: double compatible, ABO incompatible, Rh incompatible and double incompatible. The results confirm the exponential growth of serum placental isoenzyme as a function of gestational age and show that the moment of appearance of the placental isoenzyme is six weeks earlier in double incompatible matings.