Pregnancy-specific changes of antipyrine pharmacokinetics correlate inversely with changes of estradiol/progesterone plasma concentration ratios.

Antipyrine pharmacokinetics as well as estradiol and progesterone concentrations were measured in plasma of 11 healthy pregnant women during the first two trimesters and again in the same patients 6 to 20 weeks after interruption of pregnancy. During pregnancy, the clearance of antipyrine increased and its half-life decreased in all but two instances. A significant inverse relationship was found between the degree of pregnancy-specific changes of plasma clearance of antipyrine and estradiol-progesterone plasma concentration ratios: high estradiol-progesterone ratios during pregnancy corresponded to decreased plasma clearance of antipyrine; low ratios of the two hormones during pregnancy corresponded to increased plasma clearance of antipyrine. The current results indicate that the balance between circulating estrogens and progesterone may be an appropriate indicator for pregnancy-specific changes of a particular hepatic drug metabolism during pregnancy.