The key is not the period; it's ovulation. A woman is most likely to become pregnant a week to 10 days after her period ends, depending on how long her period lasts, according to WebMD. This is when ovulation typically occurs.
A woman's cycle begins on the first day of a woman's period, and generally, women ovulate two weeks into their cycles. However, women can ovulate anywhere from day 11 to day 21 of their cycles, according to WebMD. Another consideration is that sperm can live in a woman's fallopian tubes for up to 7 days. In other words, the sperm can still fertilize the egg for a number of days after intercourse, according to the U.K.'s National Health Service website.