Don't assume you're safe just because your period has arrived.
Photo: Shutterstock / New Africa
If you thought the one benefit of having your period was that you can't get pregnant, you're not going to like this: You can absolutely still get pregnant on your period. (Related: The Benefits of Period Sex)
First, a quick biology lesson. Your menstrual cycle is split into three parts: the follicular phase, ovulation, and the luteal phase. The follicular phase starts on day one of your period, when you shed, then rebuild, your uterine lining. "This phase of the cycle can be short for some women and longer for others," says Karen Brodman, M.D., an ob-gyn in New York. "But it typically lasts 14 to 21 days."
Then, you ovulate (when one ovary releases an egg into your uterus). At this time, you might notice some symptoms of ovulation, such as sore breasts, increased hunger, and changes in libido.
The next phase is the luteal phase, which starts right after ovulation. Progesterone increases, priming the uterine lining for pregnancy. Unlike the follicular phase, the luteal phase of the cycle is not variable and always lasts 14 days.
When you don't get pregnant, your estrogen and progesterone levels drop, your uterus starts shedding its lining, and your period starts, Dr. Brodman says. That puts you right back at day one of your cycle.
Now, let's address why you can still get pregnant on your period:
Your cycle can vary in length.
"A normal cycle lasts anywhere between 24 and 38 days, most commonly 28 to 35 days," Dr. Brodman says. "Some women have the same cycle interval like a clock, but others find their cycle interval is less predictable."
Because the luteal phase is always 14 days, changes in the length of the follicular phase are what change the length of your whole cycle. "A short cycle has a short follicular phase and a long cycle has a long follicular phase," Dr. Brodman says. And because the length of your follicular phase changes, that means ovulation isn't always as predictable.
"If you have a short cycle, you may actually be ovulating on day seven or eight of your cycle. And if your period bleeding lasts a long time—say, seven or eight days—then you might conceive even though you're technically still on your period," Dr. Brodman says. Plus, "even if you always have predictable periods, from time to time you may have an early or a late ovulation." That's why using the "rhythm method" as contraception doesn't always work. And you won't really know, since you'll just have your normal period.
Sperm stay in your uterus longer than you think.
It's also important to remember that ovulation isn't a five-minute window of opportunity for pregnancy. You're the most fertile for about five to seven days around your time of ovulation, Dr. Brodman says, and an egg can even be fertilized up to 12 to 24 hours after you ovulate. Not to mention, sperm can live for three to five days in your uterus. So even if you have sex toward the end of your period and don't ovulate for another few days, sperm could still be waiting to fertilize that egg.
You're actually spotting.
If you have spotting mid-cycle (which sometimes happens as your hormones change) and mistake it for your period, you could be having sex smack dab in the middle of your ovulation period. (FYI, you should try tracking your cycle on a period tracking app.)
You know where this is going: Practice safe sex every. damn. time. "If you're using reliable contraception (pills, ring, IUD, condom, Nexplanon), only then can you have sex with your period without conceiving," Dr. Brodman says.