The chances of getting pregnant while on your period are low, but pregnancy is still possible.
The answer lies in understanding the fertile window. For people who are trying to get pregnant, menstruation is not the best time to conceive. Getting pregnant depends on ovulation, which happens when an egg is released from the ovary and moves toward the uterus. The day of ovulation varies from one person to another and from cycle to cycle.
Although the chances of getting pregnant while on your period are low, there are exceptions. For example, it is possible to conceive during a period if ovulation occurs early in your cycle or if your periods last longer than five days.
The chances of becoming pregnant during menstruation are higher for people with shorter monthly cycles. Shorter cycles (21–24 days or fewer) mean ovulation occurs early in the cycle. Because sperm can live for up to five days inside your body, having sex near the end of a period could lead to pregnancy if ovulation occurs early (within that five-day sperm survival window).
Generally, most people ovulate sometime between days 10 and 17 of their menstrual cycle. The earlier ovulation occurs, the earlier the fertile window will start. The fertile window begins five days before ovulation. If ovulation occurs on day 14 of a cycle, then the fertile window starts on day nine. But if ovulation occurs on day eight of a cycle, the fertile window begins on day three — possibly in the middle of a period.
Not all fertile days are equal. The days closer to ovulation have higher chances of pregnancy than the ones further from ovulation.
Having unprotected sex a day before ovulation typically results in about a 30-percent chance of pregnancy. Having sex five days before ovulation involves about a 10-percent chance of pregnancy. This means that even if ovulation occurs early enough to start in the last days of your period, the possibilities of becoming pregnant on these days are not high.