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

What is the key mechanism of action of progesterone-only oral contraceptives to prevent pregnancy?

1. Thickening of the endometrium

2. Inhibition of fertilization

3. Inhibition of ovulation

4. Alteration of fallopian tube secretions

The correct answer is: Alteration of fallopian tube secretions

The key mechanism of action of progesterone-only oral contraceptives (commonly known as the "mini-pill") in preventing pregnancy is primarily through alteration of fallopian tube secretions. By enhancing the viscosity of the cervical mucus, these contraceptives create a barrier that makes it more difficult for sperm to reach the egg. This thickened mucus prevents sperm passage and, combined with changes in the endometrial lining, creates an environment less conducive to implantation should fertilization occur. While other actions, such as the inhibition of ovulation and changes to the endometrium, can occur, they are not as prominent with the progesterone-only method as they are with combined oral contraceptives. The primary focus of progesterone-only contraception is the modification of the cervical mucus and secretions within the fallopian tubes, reducing the likelihood of sperm encountering an egg. Understanding this mechanism is crucial for comprehending how different types of oral contraceptives operate, especially in contexts where estrogen is contraindicated or not desired for use.