How do birth control pills work?

Reviewed by Yoshinori Abe, MD

Internal Medicine

Birth control pills prevent pregnancy in three main ways: they stop ovulation (the release of eggs), thicken cervical mucus to block sperm, and thin the uterine lining to prevent implantation. These combined hormonal effects make birth control pills over 99% effective when taken correctly.

If you're experiencing side effects, unusual symptoms, or aren't sure whether your birth control is working as it should, understanding your body is the first step to making informed decisions. Take a free, instant, online symptom check to get personalized insights and guidance on your next steps—no appointment or sign-up required.

Reviewed for medical accuracy: 07/20/2026

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Explanation

Birth control pills are a type of medicine that helps prevent pregnancy. They work mainly in two ways:

Stopping Ovulation: Birth control pills stop the ovaries from releasing eggs. Without an egg, there is nothing for sperm to join with, so pregnancy cannot happen.
Changing the Uterus and Mucus: The pills also make changes in the womb (uterus) and the mucus around the cervix. These changes make it harder for sperm to reach any eggs that might be released.

These pills contain hormones similar to the ones our bodies naturally make. By taking the pill every day, these hormones stay at levels that help stop pregnancy. They are safe for most people, but if you're experiencing any symptoms or concerns about starting birth control, you can use Ubie's free AI symptom checker to better understand your health before talking to a healthcare provider about whether they are a good choice for you.

(References)

Rivera R, Yacobson I, & Grimes D. (1999). The mechanism of action of hormonal contraceptives and .... American journal of obstetrics and gynecology, 10561657.
Frye CA. (2006). An overview of oral contraceptives: mechanism of action .... Neurology, 16567739.
Sondheimer SJ. (2008). Oral contraceptives: mechanism of action, dosing, safety, .... Cutis, 18338654.

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