The mechanism of action of hormonal contraceptives and intrauterine contraceptive devices

Abstract

Modern hormonal contraceptives and intrauterine contraceptive devices have multiple biologic effects. Some of them may be the primary mechanism of contraceptive action, whereas others are secondary. For combined oral contraceptives and progestin-only methods, the main mechanisms are ovulation inhibition and changes in the cervical mucus that inhibit sperm penetration. The hormonal methods, particularly the low-dose progestin-only products and emergency contraceptive pills, have effects on the endometrium that, theoretically, could affect implantation. However, no scientific evidence indicates that prevention of implantation actually results from the use of these methods. Once pregnancy begins, none of these methods has an abortifacient action. The precise mechanism of intrauterine contraceptive devices is unclear. Current evidence indicates they exert their primary effect before fertilization, reducing the opportunity of sperm to fertilize an ovum.

PIP: The mechanism of action of contraceptive method is essential for the development of new methods. It also influences cultural and individual acceptability of a contraceptive method. Modern hormonal contraceptives and intrauterine contraceptive devices have multiple biologic effects. Some of them may be the primary mechanism of contraceptive action, whereas others are secondary. For the combined oral contraceptives and progestin-only methods, the main mechanism of action are the inhibition of follicular development, ovulation, and as consequence, corpus luteum formation. Further, it is also involved in the alteration of the cervical mucus that inhibit sperm penetration. For hormonal methods, particularly the low-dose progestin-only products and emergency contraceptive pills have effects on the endometrium that, theoretically, could affect implantation. However, no scientific evidence will indicate that prevention of implantation actually results from the use of these methods. Once implantation has taken place, none of these methods are effective and pregnancy proceeds normally. The precise mechanism of IUDs remains unclear because of difficulties in carrying out relevant investigations in humans and the limitations of extrapolating findings from animal studies. However, several studies evidenced that IUDs exert their primary effect before fertilization, by impeding the ascent of sperm to the fallopian tubes or by reducing the ability of sperm to fertilize an ovum.