Pregnancy can only occur in the few days following ovulation (the release of an egg). This usually takes place at some point in the middle of a woman's menstrual cycle, between her periods. Unfortunately, women have no definite way of knowing exactly when they are ovulating, so there is no guaranteed "safe" time to have unprotected sex. Sperm can survive inside the body for several days while they wait for an egg to be released, and the egg takes several days to travel to the uterus, meaning a woman can potentially become pregnant over quite a long period of time. This is why even unprotected sex during a woman's period can sometimes result in pregnancy. If she has irregular periods (as many girls do in the first few years of menstruation), "safe" days can be particularly difficult to predict. Some couples do use the so-called 'rhythm' method as a form of contraception (i.e. only having sex on certain days), but the success rate is not high, and it also offers no protection from STDs such as HIV
may be u r thinking to ask whether there are chances of becoming pregnant from a single act of sex
The likelihood of becoming pregnant from a single act of unprotected sex (for example, from a one night stand) varies from person to person, and also depends on the stage of a woman's menstrual cycle. The probability is highest around the time of ovulation (when the egg is released), when, on average, up to one third of women will become pregnant from having sex once.
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