Postcoital antifertility agents.
Investigators have studied a variety of agents to determine their usefulness in postcoital antifertility. To determine the mechanism of action of postcoitally administered estrogen, 2 groups of regularly ovulating volunteers were given 25mg of diethylstilbestrol for 5 days, beginning on Day 1 of basal temperature rise. Blood samples were taken from 1 group every 1-3 days to measure the levels of plasma progesterone. The other group had endometrial biopsies on the fifth day after the basal temperature rise about the time implantantion would obcur. Preliminary results showed lowered levels of plasma progesterone, and biopsy on cycle Day 20 showed endometrium resembling that expected on Day 16. Decreasing levels of circulating progesterone prevent the usual rise in endometrial carwonic anhydrase, inhibiting blastocyst implantation. Reports of adenocarcinoma in female offspring of mothers who have taken diethylstilbestrol in early pregnancy lead authors to suggest use of ethinyl estradiol 2 to 3mg daily starting within 48 hours of coitus when postcoital contraception is desired. However, side effects of nausea and vomiting restrict its use to isolated instances and it should not be used routinely.