How do birth control pills work?
There are different types of birth control pills, but most types of hormonal birth control work by inhibiting your ovulation. If your ovaries don’t release an egg each month, you cannot get pregnant.
Birth control pills typically contain estrogen and progesterone. Some pills contain only progesterone. These hormones both work to change your natural menstrual cycle and stop ovulation, although 40 percent of people who use the minipill (progesterone-only birth control) continue to ovulate.
With most combined birth control prescriptions, you take active pills for 21 days and then placebo pills (or no pills) for seven days. These seven days are known as the “rest week.” Even though you’re not taking any hormones on these days, the pill is still working to prevent pregnancy. You usually get your monthly bleeding during these last seven days, but it’s withdrawal bleeding, not a real period. There are some oral contraceptives that have 24 active pills and four placebo pills. And for progesterone-only pills, you typically take them for 28 days straight and then immediately start the next pack.
Birth control also prevents pregnancy by thickening your cervical mucus, which makes it harder for sperm to reach your uterus. It also makes your endometrial lining thinner so it’s less likely to support a fertilized egg.