A lot of people think that if a girl has sex during her period, she can’t get pregnant. But it is possible for a girl to get pregnant while she is bleeding. This can happen for a couple of reasons:
Not all vaginal bleeding is the result of a menstrual period. Sometimes a girl will have a small amount of vaginal bleeding at the time of ovulation — the time when she is most fertile. During ovulation, an egg is released from one of the ovaries and travels down a fallopian tube to the uterus. It’s common for girls who are ovulating to have some vaginal bleeding that can be mistaken for a period.
Sometimes ovulation can occur before the bleeding from a girl’s period has stopped, or it may occur within a few days after her period is over. Sperm can fertilize an egg for several days after ejaculation. So in both cases, having sex before the period is finished can result in pregnancy.
Having unprotected sex at any time is very risky. Along with the chance of becoming pregnant, there is also the risk of getting a sexually transmitted disease (STD), such as chlamydia, genital warts, or HIV. The only surefire way to prevent pregnancy and STDs is abstinence. If you do have sex, use a condom every time to protect against STDs. And talk to your doctor about additional forms of contraception.