USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

How do estrogen-containing oral contraceptives primarily inhibit ovulation?

1. By thickening cervical mucus

2. By altering motility of fallopian tubes

3. By inhibiting implantation of fertilized embryo

4. By stabilizing FSH and LH secretion

The correct answer is: By stabilizing FSH and LH secretion

Estrogen-containing oral contraceptives primarily inhibit ovulation through the stabilization of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secretion. When these contraceptives are taken, they provide a steady level of estrogen in the bloodstream. This consistent level of estrogen helps to prevent the mid-cycle surge of LH that is necessary for ovulation to occur. By stabilizing the secretion of these hormones, the contraceptive effectively inhibits the follicular development and the subsequent release of an egg from the ovary. This mechanism is crucial as the LH surge is a key trigger for the ovulatory process. Without this surge, ovulation does not occur, and thus the chances of fertilization by sperm are significantly reduced. Moreover, the hormonal feedback mechanism plays a vital role in the menstrual cycle, and by keeping FSH and LH levels stable, estrogen-containing contraceptives exert an effective means of preventing pregnancy. Additionally, while the thickening of cervical mucus and alterations in the motility of fallopian tubes can also contribute to the contraceptive effect, these actions are secondary to the primary mechanism of ovulation inhibition. Implantation inhibition is not a direct effect of estrogen but rather a more complex consequence related to hormonal changes after ovulation