A chart showing the days you are the least likely to get pregnant — including right before your period

• It is very unlikely that you would get pregnant if you have sex the day before your period.
• It's also very unlikely that you would get pregnant during your period or one to two days after it.

Whether you're hoping to conceive or trying to avoid unwanted pregnancy, it's helpful to know when you're most fertile versus when you're least likely to get pregnant. For example, you may wonder if you have a high chance of getting pregnant the day before your period.

Although everyone's menstrual cycle is unique, the phases of the cycle are the same, and there are specific times when you're most likely to conceive.

Here's what you need to know about whether or not you can get pregnant the day before your period, and when your most fertile time of the month is.

Is it possible to get pregnant the day before your period?

"It isn't likely that a woman would get pregnant right before her period," says Dr. Jane Frederick, an OB-GYN and reproductive endocrinologist at HRC Fertility, an Affiliate of Keck Medicine of USC.

This is because there's a narrow window of time, about five or six days of the month, where you're most likely to get pregnant. This is due to ovulation, which is when your body releases an egg from your ovary.

Although ovulation only occurs for up to one day, sperm can live in the body for up to five days, which is why your chances of conceiving during the days surrounding ovulation are high.

"This is the fertility window, and it usually occurs about two weeks before your period, in the middle of the menstrual cycle," Frederick says. By the time your period comes up, that egg is long gone.

In fact, if you have a regular cycle and you have unprotected sex the day before your period, you likely don't need to take emergency contraception since the chances of pregnancy are so low, Frederick says.

Chances of getting pregnant based on your cycle

The chances of getting pregnant vary based on where you are in your cycle.

Based on a regular 28-day cycle, here are your chances of getting pregnant surrounding your period, according to Dr. Alice Sutton, an OB-GYN at UC San Diego Health.

Exceptions to this guidance

The big caveat is if you have irregular periods. In this case, you might not know when you ovulate, says Sutton.

Sutton says your period is considered irregular if your cycle is shorter than 21 days, longer than 35 days, or if bleeding lasts longer than seven days. Additionally, the cycle length may vary by cycle, leaving your periods quite unpredictable.

Some situations that can lead to irregular periods are:

• Polycystic ovarian syndrome (PCOS)
• Uterine polyps
• Uterine fibroids
• Stress
• Eating disorders
• Endometriosis
• Side effects of certain medications

Or, Sutton says you might have a condition or infection that results in spotting or bleeding –– but not a true period. In this case, it can be difficult to determine when you're ovulating or having a period bleed.

Sutton says if your periods are irregular and you aren't sure if your bleeding is a true period, the likelihood of getting pregnant is higher.

Another situation where pregnancy right before your period could occur is if you have a short cycle where you might ovulate more frequently, says Frederick. Keep in mind the average cycle is considered 28 days.

However, if your cycle is irregular or on the shorter side, Sutton says it's best to contact your OB-GYN for guidance on whether or not you should take emergency contraception, weighing any pros and cons and taking your medical history into account. Your doctor can help you determine the best course of action.

"If they wanted to be absolutely sure, if they had any doubts about when their period was, or if that bleeding was really a period, their risk-benefit analysis might be different, and they might decide to take [emergency contraception] anyway, just in case," Sutton says.

Insider's takeaway

Your chances of getting pregnant the day before your period are extremely slim if you have a regular, predictable cycle. However, your chances are higher if your cycles are irregular, you have irregular bleeding or spotting, or if you have a short cycle. Everybody is different, and your ovulation time and fertile window may vary.

If you have unprotected sex the day before your period and do not want to get pregnant, contact your doctor for advice on what to do about emergency contraception to determine the best course of action.