What is the mechanism of action of the etonogestrel implant?

• A. Inhibition of endometrial growth only
• B. It primarily prevents implantation by destroying the endometrium
• C. Suppression of ovulation with thickened cervical mucus and endometrial changes
• D. Acts as a barrier to sperm

Answer: (C)

The main idea is that the etonogestrel implant works through hormonal effects that prevent pregnancy on several levels, with ovulation suppression being central. Etonogestrel, a progestin released continuously by the implant, largely blocks the midcycle LH surge, which prevents ovulation in most cycles. Even when ovulation does occur in some cycles, the thickened cervical mucus created by the progestin makes it harder for sperm to reach the uterus, and the endometrium undergoes changes that make the lining less receptive to implantation. This combination—less ovulation, harder sperm passage, and a less favorable endometrium—provides effective contraception.

Why the other ideas aren’t the best fit: focusing only on endometrial growth would miss the important role of ovulation suppression. Suggesting it prevents implantation by destroying the endometrium mischaracterizes the endometrial changes, which tend to thin or become atypical rather than being destroyed. Describing it as a barrier to sperm implies a physical barrier, which isn’t how this implant works. The implant’s effectiveness mainly comes from its progestin-driven suppression of ovulation plus changes in cervical mucus and the endometrium, not from a mechanical barrier or endometrial destruction alone.