What are the chances of getting pregnant if I have sex the day before my period?

In order to get pregnant, a sperm must meet and fertilize an egg, so any factors that affect this also affect your fertility. These include: the timing of ovulation, how long the egg lasts, whether there are any sperm present and how long sperm can live inside you.

Women who have regular periods can easily calculate when ovulation occurs. Ovulation almost always happens 14 days before you expect your period. For a woman who has 28 day cycles, that would be in the exact middle of the cycle, 14 days after the last period and 14 days before the next one. Women who have 30 day cycles can expect to ovulate 16 days after the last period, which is 14 days before the next one. Therefore, it is not possible to get pregnant on the day before you get your period, the last day of your cycle.

There is an important exception to this, however. Women who have irregular cycles cannot predict when will get their next period. Furthermore, many women who have irregular periods also have episodes of irregular bleeding that are not periods, even though they look like periods. Since you don't know when to expect your period, and you don't know if an episode of bleeding is actually your period, it is possible to get pregnant on what appears to be the day before your period.