Myth: A woman can't get pregnant during her period.

While a woman is unlikely to conceive during menstruation, "nothing, when it comes to pregnancy, is impossible," said Aaron Carroll of Indiana University and co-author of Don't Swallow Your Gum: Myths, Half-truths and Outright Lies About Your Body and Health (St. Martin's Griffin, 2009).

Once inside a woman, sperm can wait for an egg for up to a week. Ovulation can occur soon after, or even during, the bleeding phase of a woman's menstrual cycle, giving patient sperm the chance to get lucky. The timing method of birth control doesn't work well, Carroll said, agreeing that couples who practice it are often called something they likely were trying to avoid: parents.