What role does progestin play in progestin-only pills?

Progestin in progestin-only pills primarily functions to thicken cervical mucus. By increasing the viscosity of the cervical mucus, progestin effectively creates a barrier that inhibits sperm from reaching an egg, thereby preventing fertilization. This mechanism is crucial for the contraceptive effectiveness of progestin-only pills.

In addition to this primary action, progestin can also introduce other hormonal changes that may contribute to the overall efficacy of the contraceptive method, such as altering the endometrial lining to make it less suitable for implantation. However, the key role of progestin in this context is to thicken cervical mucus, making it the best answer in this scenario.

Other potential roles attributed to progestin, such as expanding the uterine lining or regulating menstrual flow, do not align with the primary function of progestin-only pills in contraceptive use. The primary goal of these pills is to prevent pregnancy, which is most effectively achieved through the thickening of cervical mucus.