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How does progestin make it harder for sperm to reach an egg?

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Blister pack of 28 oral contraceptive pills arranged in four rows, with 21 white active pills followed by 7 orange inactive pills. Arrows indicate the order for taking pills. — birth control A 28-day package of birth control pills.

Progestin, a synthetic form of the hormone progesterone, prevents pregnancy through several mechanisms. Regarding its effect on sperm, progestin causes the mucus lining the cervix to thicken.123

This thickened cervical mucus acts as a physical barrier, making it significantly more difficult for sperm to enter the uterus and reach an egg. Additionally, in some preparations, progestin makes the cervical environment more acidic, rendering it hostile to sperm. By creating these physical and chemical obstacles, progestin effectively reduces the likelihood of fertilization.132