Amniotic fluid alkaline ribonuclease activity: an indicator of foetal growth retardation and anencephaly.

Amniotic fluid alkaline ribonuclease (RNase) activity was measured in 55 normal and 29 neural tube defect pregnancies. The pregnancies which resulted in neural tube defects gave a mean value of 2,500 U/l with a SD of 600 compared with a mean activity of 1,600 U/l (SD 350) from the control normal pregnancies. The mean amniotic fluid RNase activity of 15 anencephalic pregnancies was 2,600 U/l (SD 600), compared with 1,600 U/l (SD 350), for 28 gestational age-matched controls. Highest RNase activities appear to be related to anencephaly.