Maternal and foetal corticosterone levels during late pregnancy in rats

Adrenal and plasma corticosterone levels were determined in rat foetuses and in intact or adrenalectomized mothers during late pregnancy. Foetal adrenal and plasma corticosterone concentrations reached a peak on day 19 of pregnancy, while maternal plasma corticosterone increased on day 18 and remained high until parturition. From day 18, mothers adrenalectomized on day 14 had corticosterone levels similar to those of intact pregnant rats. At every stage of gestation (except day 21) plasma corticosterone levels were higher in the foetuses than in the mothers. The corticosterone concentration in the maternal plasma correlated with the number of live foetuses during the last 3 days of gestation. These results suggest that corticosterone can cross the placenta from foetus to mother as early as day 18 and that the foetus contributes to the maternal corticosterone pool after day 18.