Oral contraception: Mechanism of action

The combination of steroid hormones in oral contraceptives acts both centrally and peripherally to alter normal reproductive function. Ovulation is inhibited through suppression of gonadotropin release. Follicuar maturation is impaired, although there is evidence that follicular atresia continues. The incidence of ovulation in women taking oral contraceptives is less than 2%. As the number of missed days of oral contraceptive use increases, the risk of ovulation rises. However, even in the face of occasional ovulation, the peripheral changes that occur secondary to chronic exposure to the progestin component of the oral contraceptive make both fertilization and implantation unlikely events. Hence, while the risk of ovulation increases with the use of formulations that contain progesterone only, and in the newer low dose contraceptives, the risk of pregnancy does not increase markedly.