Although the possibility of getting pregnant right after your period may be lower for some women than others, the reality is that pregnancy can occur at any time during your menstrual cycle. To understand why this is possible it's important to first understand how pregnancy occurs.
The number of days from the first day of your period to the first day of your next period is the length of one menstrual cycle. The average menstrual cycle length is 28 days; however, this can vary from less than 21 days to more than 40 days. Ovulation always occurs 14 days before the first day of your period. This means if your menstrual cycle length is 30 days, ovulation occurs 17 days after the first day of your period. If your cycle length is 24 days, ovulation occurs 11 days after the first day of your period.
Sperm can live in fertile cervical mucus for about five days, which means you are most likely to become pregnant approximately five days before and up to the day of ovulation. So, if your cycle length is 24 days and your period lasts 10 days, you can become pregnant if you have intercourse on or after the sixth or seventh day of your period.
The length of your menstrual cycle can be altered by stress, weight loss, weight gain, diet, and exercise. In fact, many women have irregular menstrual cycle lengths which can make it difficult to accurately predict when they will ovulate. This is because the timing of ovulation is measured backwards from the first day of your period. Since it's not always possible to know if your next menstrual cycle will start earlier or later than usual, it's not always possible to accurately predict when you will ovulate. For example, if your cycle is usually 28 days long and one month it happens to be 24 days, you may become pregnant during or immediately after your period.
In addition to irregular cycle lengths many women may experience irregular bleeding which can occur between your regular periods or can cause a prolonged period. Irregular bleeding can occur when you ovulate or can be due to infections, benign growths in the uterus or cervix, cancer, or if you have had a previous cesarean delivery. If irregular bleeding occurs between periods you might mistake it for a regular period and miscalculate your date of ovulation. If you experience a prolonged period it is very possible that you may ovulate while you're still bleeding. Having unprotected intercourse during either of these times can result in pregnancy.