Effect of fetal decapitation on unbound rat plasma corticosterone concentration at the end of pregnancy.

In normal fetuses from day 16 to 21 of pregnancy, unbound plasma corticosterone concentration increases slowly from day 16 to 17, and rapidly from day 17 to 19, remaining high until birth. Fetal decapitation on day 18 produces a fall in unbound plasma corticosterone only when the mother is adrenalectomized. The ability of corticosterone to cross the placental barrier from mother to fetus or inversely is discussed.