Can you get pregnant right before your period? This is a frequently-Googled period question, the answer to which hinges on hope or relief depending on whether or not you’re aiming to conceive.

So let’s break it down. But first let’s clarify some words:

• “Getting pregnant” = sperm + egg. Pregnancy starts when your egg fuses with a sperm cell—a process called fertilization. It’s not technically considered a ‘pregnancy’ until the fertilized egg nestles into the lining of the uterus and starts growing—a process called implantation.
• “Right before” = sometime during your luteal phase—the 10 to 16 days between ovulation and your period, when PMS happens.
• “Your period” = the shedding of your uterine lining, indicating that no fertilized egg has cozied into it, and you aren’t pregnant.

Can you get pregnant right before your period?

It’s very unlikely. (*but there are exceptions, see below.)

You can only get pregnant during your fertile window—the six or so days per menstrual cycle that you’re fertile. Your fertile window occurs before and during ovulation—when your ovaries release an egg.

It only takes a few minutes to release an egg from your ovary. Once released, your egg has about 24 hours to live. Your fertile window is six days long, rather than just those 24 hours, because sperm can survive in the uterus for up to five days, ready to fertilize an egg should one be ovulated.

You only ovulate once per menstrual cycle. (In rare cases, you may release more than one egg per ovulatory episode, and may conceive twins. But then you cannot ovulate again until next your cycle.)

It’s typical to ovulate around day 14 of your cycle (about 14 days after the first day of your period). But this is just the norm, not the rule.

Ovulation is always followed by a luteal phase lasting about 2 weeks, followed by a period (if you’re not pregnant). Even for people with cycles longer than the typical 28 days, the luteal phase still lasts about 2 weeks, and a longer follicular phase—the phase before ovulation—accounts for the extra days.

To estimate your ovulation day, identify the day of your cycle you started bleeding heavily (spotting doesn’t count), then subtract 14 days. The closer your ovulation day to the day you had unprotected sex, the greater your chances of getting pregnant.

But if you’re concerned (or hopeful) about getting pregnant a few days before your period, odds are ovulation is long over, your fertile window has closed, and the likelihood of pregnancy is low.

Say you had unprotected sex on day 21 of your cycle: about 7 days after your ovulation day, and about 7 days before your period started. By one study’s estimate, there’s a 5% chance you were fertile on that day. (But just because you were fertile doesn’t mean your partner was as well.) The odds continue to drop off each day after that.

And a 5% chance of getting pregnant on day 21 of your cycle is a conservative estimate compared what other researchers have found:

Ovulation doesn’t happen in all menstrual cycles. Anovulatory cycles—menstrual cycles in which ovulation did not occur, are relatively common; one study estimates about a third of otherwise “normal” menstrual cycles are anovulatory. You cannot get pregnant during an anovulatory cycle, as it offers no egg to fertilize.

No matter your chances of pregnancy, conventional at-home pregnancy tests (the ones you pee on) are a fairly reliable way to know for sure if you’re pregnant or not.

According to the Mayo Clinic, “Many home pregnancy tests claim to be accurate as early as the first day of a missed period—or even before. You’re likely to get more accurate results, however, if you wait until after the first day of your missed period.”

Waiting a day yields better results because most at-home pregnancy tests are looking for Human Chorionic Gonadotropin (HCG)—a hormone your body makes after the fertilized egg attaches to the wall of the uterus. The amount of HCG in your urine rises quickly after implantation, making pregnancy tests more and more accurate with each passing day.

Exceptions: When you CAN get pregnant right before your period

1. If what you think is a period isn’t actually a period. It’s possible to mistake your period for ovulation bleeding, especially if it comes with period-like symptoms. Should you mistake an ovulation bleed for a period, “right before your period” (or at least, what you thought was a period) overlaps with your fertile window, and the likelihood of getting pregnant is quite high.

2. If you have a long fertile window. Ovulation typically occurs around day 14 of a 28-day menstrual cycle, but everyone’s cycle is different. One study found that “most women reach their fertile window earlier and others much later” than day 14.

It’s rare, but you may have a uniquely long, or uniquely late-arriving fertile window. This can occur naturally, or be a result of stress, medications, thyroid disorders, or other conditions. No matter when it occurs, ovulation always presents the possibility of getting pregnant, even if very late in your cycle.

Just note that after fertilization, the egg still needs to be implanted into the uterine wall. That can take a week or more. If you fertilize an egg during a fertile window that is very close to your period, the odds of a failed implantation (and thus a non-pregnancy) increase because the uterine lining is shed during your period, potentially shedding the fertilized egg along with it.

3. If you have irregular cycles. If your period doesn’t arrive regularly each month (if you sometimes get two periods per month, or go a month or more without a period, for example), it can be very difficult to determine when you ovulated, and thus when your fertile window was. At-home ovulation tests can help take the guesswork out of when you’re most likely to become pregnant, especially for those with irregular cycles.

Courtney Mayszak, RDN, LDN is the co-founder of De Lune, who offer an all-natural line of menstrual relief products.

Shop De Lune here!