Can You Get Pregnant On Your Period? It Depends on Your Cycle Length.
Can you get pregnant on your period? It depends on when ovulation occurs in your cycle. The day of ovulation varies from woman to woman, and in the same woman from cycle to cycle. You can only get pregnant from having sex on your period if you ovulate early in your cycle, or if your period lasts much longer than the average five days.
Getting pregnant on your period is unlikely for most women, but it is not impossible. The answer lies in understanding a woman’s fertile window.
When in your cycle can you get pregnant?
While a man is fertile every day, a woman’s fertile window only lasts six days per cycle. Why only six days? The length of the fertile window is determined by the lifespan of the egg, plus the lifespan of sperm. Sperm can survive for up to five days in the fallopian tubes when conditions are favorable. Once the egg is released, it must be fertilized by sperm within 12 – 24 hours or else it will disintegrate. Add up the lifespan of the sperm and the lifespan of the egg, and you get a fertile window of six days per cycle, starting from five days before ovulation and ending on the day of ovulation.
When in your cycle do you ovulate? In a textbook cycle, ovulation occurs on day 14—that is, 14 days after the first day of menstruation. In this textbook cycle, the fertile window—the days on which sex has the potential to result in pregnancy—lasts from day 9 – 14. In this scenario, unless a woman had an unusually long menstrual cycle, it would be extremely unlikely, if not impossible, for her to get pregnant from sex during her period.
Can you get pregnant on your period?
Of course, the female body is not a machine. Most women don’t ovulate on day 14. In fact, only 30 percent of women have cycles where the fertile window falls entirely within the days identified by clinical guidelines[1.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC27529/].
The earlier you ovulate, the earlier you will hit your fertile window, since your fertile window starts five days before you ovulate. If you ovulate on day 14 of your cycle, for example, then your fertile window begins on day nine. But if you ovulate on day eight of your cycle, your fertile window begins on day three—right in the middle of your period.
One thing to remember is that all fertile days are not created equal. The days closer to ovulation carry a higher chance of pregnancy than the days further from ovulation. Having sex the day before ovulation has, in the best case scenario, about a 30 percent chance of resulting in pregnancy. Having sex five days before ovulation has a less than five percent chance of resulting in pregnancy[2.https://academic.oup.com/humrep/article/14/7/1835/2913179/Day-specific-probabilities-of-clinical-pregnancy]. That means that even if you ovulate early enough in your cycle for your fertile window to begin in the last days of your period, your chances of getting pregnant on those days are not high.
Is getting pregnant on your period likely? No.
But just how likely is it to ovulate early enough in your cycle that your fertile window begins during menstruation? Women whose cycles are usually 27 days or less are more likely to have fertile windows that begin during their periods[3.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC27529/]. An estimated two percent of women are in the fertile window by the fourth day of their cycle, compared to 17 percent by the seventh day[4.ibid].