Pregnancy is what happens when a guy’s sperm meets up with a woman’s egg and the fertilized egg sticks to the lining of the woman's uterus. So any type of sex where semen gets inside the vagina could result in a pregnancy—even if a guy’s penis is near but not inside the woman’s vagina when he ejaculates. It doesn’t matter what position you’re in—reverse cowgirl, missionary, standing, whatever. Sperm + egg + sticking to the uterus = pregnancy.
But just how likely is a woman to get pregnant from unprotected sex? It depends on where she is in her cycle.
Can I really get pregnant during my period?
The short answer is YES! If you have regular periods, you are less likely to get pregnant while on your period than at other times in your cycle—but that doesn’t mean it’s impossible. How do I know it’s not impossible? There have been tests of a fertility awareness method called “Standard Days”—where women have unprotected sex at specific times in their cycle, including while on their period. About one in four women using this method get pregnant. The bottom line is that if you really don’t want to be pregnant, use birth control every time you have sex.
How can I tell if my period is “regular”?
A regular period is one that comes at about the same time each month. Most women have cycles lasting between 25 and 28 days, but there is a range. For some women, their regular cycle can be up to 35 days!
Here’s how to figure out whether you have a regular period:
Step 1: Start by counting the first day that your period begins as day one.
Step 2: Count each day after that until you get your period again.
Step 3: Start over with day one. That’s it!
If you count for a few cycles and all your periods are about the same number of days, then you have a regular period. Not every cycle will be the exact same number of days, but don’t sweat it—it’s normal to have a few days more or less each month.
Your period is irregular if it happens at very different times from one cycle to the next—for example, 24 days, then 32 days, then 27 days. If two or more months go by and you don’t have a period, that’s also irregular. It’s common to have irregular periods, especially within the first few years of starting to menstruate. If you have an irregular period, some methods of birth control can help make it more regular.
What does ovulation have to do with it?
In general, if you have any type of period and you’re not on a birth control method that blocks the release of an egg, you ovulate. Ovulation is the release of an egg from a woman’s ovary. A woman is most likely to get pregnant if she has sex without birth control around the time it happens.
If you’ve counted the length of your cycles as described above, you can estimate when you ovulate. It usually happens about halfway through your cycle. So for a 28-day cycle, it happens around day 14.
If you’re really into learning about your body, you can find out more about when you ovulate by checking your cervical mucus and taking your temperature every morning before you get out of bed. Before you try these techniques, I highly recommend taking a class about them or talking to an experienced healthcare provider. There are also ovulation kits you can buy at a drug store to find out exactly when you’re ovulating. Another chance to pee on a stick!
How soon can you test for a pregnancy after unprotected sex?
If you’re worried about a pregnancy, it doesn’t work to take a pregnancy test the day after you had sex. Here’s why:
It takes a fertilized egg about a week to implant into the lining of the uterus. Sometimes it can take up to 12 days.
Once implantation occurs, your body begins making a pregnancy hormone called human chorionic gonadotropin (hCG).
Home pregnancy tests measure hCG in urine to determine if you’re pregnant. Clinics can measure hCG in urine or in a blood sample.
It takes a little while for there to be enough hCG to measure with any test.
All this means that you have to wait two weeks after unprotected sex to get an accurate pregnancy test result.
It's too early for a pregnancy test... Is it too late for EC?
If you’re worried about being pregnant and it's been less than five days since you had unprotected sex, there’s one important thing you can do right away: use emergency contraception (EC).
Emergency contraceptive pills block ovulation up to 5 days after unprotected sex, although they are more effective the sooner you take them.
If you want emergency contraception plus long-lasting and highly effective birth control, consider a copper IUD. It prevents pregnancy when inserted up to 5 days after unprotected sex, and goes on to provide almost 100% effective birth control for up to 12 years.
As always, if you have questions or want more details, talk to your health care provider!