Heat-stable alkaline phosphatase as an index of placental function

In spite of the fact that amniocentesis is of value in Rh isoimmunization and urinary estriol excretion is of value in some complicated pregnancies, a great need still exists for a simple, inexpensive means of assessing placental function. The placenta is the source of most of an isoenzyme of alkaline phosphatase which is heat stable and therefore easily identified in the laboratory. Determination of this fraction has been carried out 500 times in 120 pregnant women in an attempt to correlate values with placental function. Beyond 34 weeks' gestation a value below 3.0 King-Armstrong units or a progressive fall of serial values are ominous signs for the fetus. There is good but not perfect correlation of this test with urinary estriol excretion.