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It is less likely for your girlfriend to get pregnant, however in some minority of the women do get pregnant when they have sexual intercourse during their period. It is because not all vaginal bleeding is 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. Girls who are ovulating sometimes have some vaginal bleeding that can be mistaken for a period.
Sometimes ovulation can happen before the bleeding from a girl's period has stopped or within a few days after her period is over. All girls can ovulate at different points from cycle to cycle, making it impossible for a girl to know exactly when she is most fertile. Because sperm can fertilize an egg for 72 hours (3 days) after ejaculation, having sex during a girl's period is risky.
Chances of pregnancy during periods are minimal. However, if you strictly want to avoid pregnancy it is advisable to use an adequate birth-control method each time you have intercourse, including during periods. Though the chances of pregnancy during periods are very low, they are not entirely absent because the sperm can survive in the body for a few days and there is a small chance that an early ovulation will cause pregnancy.
So, you can go for type b emergency contraceptive pill if you are not desiring for pregnancy now.
Hope this helps. feel free to ask if you any further questions.