Can I rely on my birth control to prevent me from getting pregnant if it doesn’t stop my period?

Great question! If you use birth control as prescribed, the chance of pregnancy is close to zero. Most birth control methods work by preventing ovulation, but they don’t always impact other hormonal changes that cause a period (like Depo/the shot and hormonal IUDs). Other forms of birth control, like the pill, are designed to give something like a period because it is comforting to the user.

Here’s what I mean. A traditional period is the shedding of the lining of the uterus. Every menstrual cycle includes the buildup of fresh tissue and blood vessels on the inside of the uterus, driven by increases in estrogen and progesterone, in anticipation of a possible pregnancy. When no egg implants, the estrogen and progesterone levels slowly go down, which eventually leads to the shedding of the lining of the uterus. This is your period. The cycle starts all over again after a period.

The bleeding that occurs with monophasic birth control pill regimen is actually called “withdrawal bleeding,” not a period. That’s because the hormone pills in the pill pack (usually 21–24 days) contain a combination of hormones that both stop ovulation (progesterone) while providing a steady flow estrogen to the body (which has important roles in other parts of the body besides periods). When the estrogen and progesterone are stopped (which happens when you take the placebo pills of the birth control pack) you naturally experience some bleeding and shedding of the lining of the uterus with some mild cramping. But it’s not a full period. This is why birth control pills can also be used for heavy and painful periods. They provide a controlled level of hormones that should produce a lighter, less painful period.

Which brings us to the question — is withdrawal bleeding essential? And the answer is no. Some people find that maintaining some monthly schedule is comforting and choose to have withdrawal bleeding every month. Other folks take birth control “continuously,” meaning they just toss out the placebo pills and start the next pill pack. There is then no withdrawal bleeding, and that’s okay! Ovulation should remain suppressed while you take your pills every day, whether or not you have withdrawal bleeding, and that should provide great protection against pregnancy.