PIP: Estrogens have been used as postcoital contraceptives with success. Of 100 women who had midcycle coitus and were given stilbesterol or ethynyl estradiol for 4 or more days, none became pregnant. Estrogen administration lowered basal body temperature, and endometrial biopsies showed retarded endometrium. Nausea was present among 40% of the women and 12% experienced breast soreness. The hypothesised mechanisms of action include: accelerated ovum transport, change in stickiness of the blastocyte, and changes in the implantation site, all leading to the prevention of implantation.