Anywhere from 3-7 days. Periods are when the lining of the uterus sheds and comes out your vagina. In more medical terms, the lining of the uterus builds up during your menstrual cycle because of the hormone progesterone. If there isn’t a constant source of progesterone (which happens with a pregnancy) the uterus realizes it doesn’t need to keep all that extra lining. And then you bleed.
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