Oral administration of micronized natural progesterone in late human pregnancy: Effects on progesterone and estrogen concentrations in the plasma, placenta, and myometrium

A single dose of micronized oral progesterone was administered to 15 pregnant women immediately prior to elective cesarean section. Levels of progesterone, 17β-estradiol, and estrone were measured in the plasma, in the placenta, and at different sites in myometrium obtained during the surgical procedure. Results were compared to those observed in a control group of women who did not receive progesterone. Progesterone levels demonstrated a marked increase in plasma and in the whole myometrium 150 minutes after administration. The levels then decreased rapidly to control values in 1 hour. The concentrations of progesterone in the placenta did not show any changes. No difference appeared in 17β-estradiol levels in the plasma or the myometrium, whereas an increase was observed in the placenta. Estrone levels did not change in the plasma, but they decreased in the myometrium and in the placenta.