If you're relying on your menstrual cycle as a form of birth control, you might be surprised to learn that you can, in fact, get pregnant during your period.

Sure, it's one of the less fertile times of the month, but under some conditions, conception could occur.

How can you get pregnant on your period?

The chances of getting pregnant when you have your period are low, but they're not zero. So how could a pregnancy happen at the same time your body is deciding it’s not pregnant?

Let’s break it down: During ovulation, your ovary releases an egg into your fallopian tube, where it can live for 24 hours awaiting fertilization before beginning its journey to the uterus. Meanwhile, the uterus is building up a lining just in case it needs to host a growing embryo.

If egg meets sperm during ovulation and implants in that lining, bingo — baby on board. If not, your body sheds both lining and egg (and … you’re bleeding).

Most women’s menstrual cycles last about 24 to 28 days, from the first day of one period to the first day of the next period. Because the egg can survive for 24 hours after ovulation and sperm can live inside a woman for up to five days, a woman can get pregnant starting five days before ovulation and ending a day after.

“Women tend to ovulate about 14 days before they get their periods,” says Mary Jane Minkin, M.D., a board-certified OB/GYN and clinical professor at Yale University School of Medicine in New Haven, Connecticut.

So if you have a 28-day cycle and a week-long period, that leaves you about a week between the last day of your period and the first day you ovulate, according to the Mayo Clinic.[1]

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But what if you have sex on the last day of your period (day seven) and you ovulate a few days early — on, say, day 12 instead of day 14? That sperm might still be hanging around, which could cause you to get pregnant, Dr. Minkin says.

Even in a woman who has a 28-day cycle, the day ovulation starts can vary from month to month.  And, says Dr. Minkin, “very few women have a totally constant cycle, every cycle.” Because of this fluctuation, you can’t guarantee that the sperm from sex during your period will be dead by the time you ovulate next.[2]

Can you get pregnant on your period if you have an irregular cycle?

Yes. If you have an irregular period, it can be almost impossible to predict when you’ll be ovulating, says What to Expect Medical Review Board Member Dr. Shannon Smith, a board-certified OB/GYN and partner at Brigham Faulkner OB/GYN Associates in Boston, Massachusetts. And if you can’t reliably predict when you’re ovulating, you could get pregnant accidentally.[3]

Here are a few types of irregular menstrual cycles, and how it's still possible to get pregnant if you fall into one of these categories:[4]

  • You have a short cycle. Some women have cycles as short as 21 days and still ovulate, which means they don’t have as much time between when their period ends and they ovulate next. And since sperm can live in your reproductive tract for a few days, if you happened to have sex during your period and then ovulated two to three days later, that egg could encounter viable sperm. Which could mean pregnancy.
  • Your periods last a long time. While your body is slowly shedding that uterine lining, the clock is ticking on ovulation — and your next egg could show up while you’re still bleeding from the previous month's period. So you’re already fertile again — which means you could get pregnant on your period.
  • You have breakthrough bleeding, or spotting, when you ovulate. Women with irregular bleeding could mistake the bleeding for their menstrual cycle, says Dr. Smith. In some cases, these breakthrough bleeding periods occur when you’re actually most fertile, since you’ve just ovulated.[5]

Sometimes, however, an irregular period can be a sign that you're not ovulating at all, which is one cause of infertility. (You can't get pregnant if you don't ovulate.) 

Breakthrough bleeding can also be a sign of hormonal abnormalities or a physical abnormality in your uterus (such as polyps, fibroids or an overly thick lining), so bring it up with your doctor even if you're not trying to get pregnant.

“Some medical conditions, like polycystic ovary syndrome, can cause irregular cycles,” says Dr. Smith. “Also, women in their teens and early 20s who have recently started their period, as well as women in perimenopause in their 40s, have more unpredictable periods due to hormone [fluctuations].”

If you have an irregular menstrual cycle and you've had trouble getting pregnant, talk to your doctor, who may want to do a further evaluation.

Can you get pregnant right before your period?

It’s not impossible, but it is unlikely — especially in women with regular, 28-day cycles. Ovulation is normally complete by day 19, a solid week-and-a-half before your next menstrual period begins.

That said, ovulation is harder to predict in women with irregular periods, which means it could happen later in the cycle. Plus, even in women with regular cycles, ovulation can vary from month to month, especially if you're stressed, traveling, have recently gained or lost weight, are ill, or recently had surgery, says Dr. Smith.

Can you get pregnant on the first day of your period?

It's possible — though not very likely — for you to get pregnant on the first day of your period, especially if you have a regular, 28-day cycle.

If you have an irregular or a shorter menstrual cycle, however, ovulation might occur much closer to day one of your period, which could lead you to get pregnant on that first day.

Can you get pregnant right after your period?

Yes, you can get pregnant right after your period. The likelihood of this happening, however, depends again on when you ovulate. In women with a 28-day, regular cycle, ovulation usually begins around day 14, which would be seven days after the end of a week-long period.

If you ovulate early, or if you have a short menstrual cycle or a long period, the amount of time between your period and ovulation could be much less. And because sperm can stick around for five days, sex only a day or two right after your period could lead to a baby.

Can you get pregnant on your period while using birth control?

It's possible to get pregnant when you're using birth control, whether you're on your period or not. (The only 100-percent-effective type of birth control is abstinence.)

The likelihood that a contraceptive will fail varies depending on the method of birth control. Here are some statistics from the Centers for Disease Control and Prevention (based on typical use):[6]

For what it's worth, tracking your monthly cycle — called "fertility awareness" — in order to avoid pregnancy isn't the most reliable birth control method; the typical failure rate may be as high as 24 percent, according to the American College of Obstetricians and Gynecologists.[8]

“Natural family planning is one of the more complex methods of contraception,” says Dr. Smith. “The more involved a method is for providing contraception, the higher the actual use failure rate because there are more steps to ‘mess up.’”

So unless you're in a committed relationship and sure you want to get pregnant at some point, doctors suggest you avoid relying on tracking your cycle as a form of birth control.

“The only people to whom I would recommend the rhythm method to is for a couple who are together, and planning a pregnancy, say, next year, but wouldn't be devastated if they conceived now,” says Dr. Minkin.

The bottom line: Although the odds are slim, you can get pregnant on your period — or any other time of the month. It's even possible (though not as likely) to get pregnant on your period if you're using birth control.

So unless you’re trying to conceive, it’s always smart to be vigilant about contraception — unless you like big surprises.