Welcome to Advanced Sex Ed. Put on your smarty pants because we’re kicking things up a notch with some higher-level sexual learnin’.
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Advanced Sex Ed: The Science of the Placebo Week
Someone asked us:
Hey PP, this is kind of a science question, but I’m wondering why birth control pills can fail if you accidentally skip a day or don’t take the pills at the same time everyday, yet I have heard it’s fine and safe to have sex when you’re on your period (during the placebo week of pills). That seems weird, how does it work?! Thanks!
First let’s talk about the menstrual cycle and fertility. During a normal menstrual cycle (without any kind of hormonal birth control), your body releases an egg from your ovaries about once a month. During that month, your body prepares for a pregnancy by building up the lining of your uterus. If the egg isn’t fertilized by sperm, then no pregnancy occurs. Your period is your body flushing out the lining of your uterus along with your unfertilized egg. All of this is directed by signals your brain sends to your body via hormones.
Each birth control pill has a small amount of hormones that send signals to your body. Except the hormones in your pills keep your eggs from leaving your ovaries, and thicken your cervical mucus, which makes it difficult for sperm to get through to an egg. It also keeps the lining in your uterus thinner (hence why taking the pill leads to lighter and more manageable periods).
We like to compare the pill to having your own personal security guard that keeps your egg from leaving your ovaries, and stops sperm from reaching your egg. In general, each pill has enough hormones to last until the next pill (plus a little bit longer as a buffer). So if we’re thinking of the pill as a guard, missing a pill means your guard might be off duty. That egg might sneak by, and any sperm that happens to be hanging around in your body from sex in the last 6 days might get the chance to reach your egg.
Your “guard” doesn’t have to be on duty during the placebo week, AKA the week you’re not taking hormones. The window for ovulation occurs during the weeks you’re taking your pill. Once that time has passed, the pill has already done its job to prevent fertilization for the month. By the time the placebo week comes around, the chance for an egg to leave your ovaries has already passed. This is also the week when you’ll get your period. So as long as you took your pills as directed that month and start your next pack on time, pregnancy is incredibly unlikely.
With the combination pill, there’s usually enough hormones in each pill to give you a bit of flex time. It’s still best to try and take it at the same time every day, but if you miss a pill you’ll usually be ok if you follow your prescription’s instructions on what to do next. The mini-pill, or progestin only pill, doesn’t have as many hormones, so it’s really important to take it at the same time every day. If you’re as little as three hours off, you could be at risk of pregnancy.
Hope you enjoyed this heaping helping of science!
-Kellie at Planned Parenthood














