Probable mode of action of oral contraceptives.

It has been shown that the gestagen component of oral contraceptives changes the composition of the cervical mucus so as to make sperm penetration difficult. Endometrial changes and changes in the normal patterns of tubal and uterine motility have been described. Ovulation is inhibited by inhibition of ovarian response to gonadotrophins or by inhibition of the formation and/or release of endogenous gonadotrophins. The "classical" pill inhibits ovulation by interfereing with the formation or release of luteinizing hormone (LH). The sequential form of oral contraception has a relatively high dose of estrogen which inhibits secretion of both LH and follicle stimulating hormone (FSH); the gestagen component causes better shedding of the endometrium. When very small doses of gestagens are given continuously, ovulation is usually not inhibited. Biochemical rather than morphological endometrial change may be most important. It is concluded that the mode of action of oral contraceptives is not completely understood. Discussion by others follows the paper.