Response of the rat fetus to acute umbilical cord occlusion: an ontogenetic adaptation?

Occlusion of the umbilical cord on days 19, 20 or 21 of gestation results in a stereotypic behavioral response by the rat fetus. The intensity of this response is diminished in older fetuses. Abrupt changes in fetal heart rate accompany the behavioral response to hypoxia. Changes in fetal activity and heart rate appear to be dissociated, however, because bradycardia immediately after umbilical cord occlusion is more pronounced in older fetuses. The possibility that these behavioral and cardiac responses to hypoxia are ontogenetic adaptations restricted to the prenatal period is discussed.