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Possible mechanisms of action of a combination of ethinylestradiol (EE) and dl-norgestrel as a postcoital contraceptive agent were studied in 12 healthy female volunteers. An oral dose of 0.1 mg of EE and 1.0 mg of dl-norgestrel was given at the predicted time of ovulation and again 12 hours later. Serum luteinizing hormone, prolactin, progesterone, 17 alpha-hydroxyprogesterone, and estradiol were measured by specific radioimmunoassays in blood samples obtained daily from the 8th day of the menstrual cycle to the 1st day of menses. Hormone profiles suggested that the medication elicited a range of individual variations in pituitary and/or ovarian responses. Histologic examination of the endometrium consistently showed significant alteration in endometrial development with a dissociation in maturation of glandular and stomal components. This postcoital contraceptive acts either by (1) suppressing ovulation or (2) disrupting luteal function by acting directly on the corpus luteum or by interfering with appropriate endometrial responses to ovarian steroids.

A combination of 1.0 mg dl-norgestrel and 0.1 mg ethinylestradiol (EE) was administered orally at 36 hours after the detection of the luteinizing hormone peak and again at 48 hours in 12 healthy volunteers with normal menstrual cycles. The effects on ovarian function were studied by comparing the daily serum levels of progesterone (P), 17 alpha-hydroxyprogesterone, and estradiol (E2) in control (placebo) and treatment cycles. Five subjects showed no significant change in the levels of these steroids but had a shortened luteal phase. The treatment significantly decreased both P and E2 levels in three subjects, while two subjects showed diminished E2 levels only. The remaining two subjects had lower P levels and fluctuating E2 patterns. Endometrial biopsies from both study cycles indicated asynchronous development of the epithelial and stromal components in the treatment cycle. These findings (abnormal luteal phase steroid levels and duration and outphased endometrial development) indicate that corpus luteum function was variously affected by the action of norgestrel-EE treatment.