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The reason that it’s not always true is because women are only fertile a few days a month, but that window is very different from woman to woman. Some women can have a 21-day cycle and some can have a 35-day cycle, and for the woman that has one week in the luteal phase (the time between ovulation and when you have your period) if she bleeds for seven days, she’s going to start ovulating while she’s still bleeding, and sperm can live for three days, so women can get pregnant if the length of their period is long.