"It's very uncommon, but it is possible since having your period doesn’t predict when you will release an egg which has to be present for sperm to begin the process of conception. Additionally, since sperm can live inside your uterus for up to 5 days, if you release an egg during these 5 days you run the risk of getting pregnant. Due to the unpredictability of your cycle, if you want to avoid pregnancy it’s important to use contraception even when you are menstruating."
"Pulling out before the man ejaculates, known as the withdrawal method, is not a foolproof method for birth control. Some ejaculate (fluid that contains sperm) may be released before the man actually climaxes. In addition, some men may not have the willpower or be able to withdraw in time."