Normal fetal physiology and behavior, and adaptive responses with hypoxemia.

The principal objective of antenatal testing is to detect fetal hypoxia before the fetus has suffered lasting harm. This article summarizes some of the mechanisms by which fetal oxygen consumption is maintained under normal conditions, and adaptations that occur in response to hypoxemia. Alterations in the fetal heart rate are prominent in the fetal response to hypoxemia and are the basis of some methods of antenatal testing. The principal mechanisms underlying these fetal heart rate changes are described. Adaptations in the distribution of blood flow that permit the fetus to deal with reduced oxygen levels are summarized. Developmental trends in fetal motility and behavior are outlined, and also how these are modified by fetal growth retardation and maternal diabetes. Fetal movements are suppressed during acute hypoxemia, but with gradually developing hypoxemia, normal movement patterns may continue until the fetus becomes acidemic. This may limit the use of these biophysical variables in antenatal testing.