Pharmacodynamics of combined estrogen-progestin oral contraceptives 3. Inhibition of ovulation

Introduction: Following a historical overview, the ovulation-inhibiting effect of various orally administered estrogen-progestin combinations (combined oral contraceptives [COCs]) are examined for their components alone or in the various combined formulations. Special emphasis is given to products containing natural estrogens. Areas covered: Inhibition of ovulation with progestins alone; estrogens alone; various progestins in combination with ethinyl estradiol; various progestins in combination with natural estrogens (estradiol, estradiol valerate, and estetrol). Expert commentary: The original idea to achieve ovulation blockage through the administration of steroid hormones involved the use a progestogen (both progesterone and its synthetic homologous). The ability of a progestin to inhibit ovulation depends on the type of compound and on its dosage and a difference of more than 20-fold in activity exists between compounds utilized today in COCs. Initially, the estrogenic component was present only because it contaminated the first progestin utilized. It was soon found that an estrogen is necessary for proper cycle control. It was also found that the estrogen acts synergistically in inhibiting ovulation. For almost half a century, most COCs contained ethinyl estradiol. Today, also natural estrogens are being employed. Inhibition of ovulation was complete with all early high dose preparations. Decreasing dosage allowed some ovarian activity to occur, occasionally leading to a mature follicle. Even in this situation, defective corpus luteum formation assured contraceptive protection.