Every month, one of your ovaries get ready to release an ovum (egg) which if fertilized by a sperm will become a baby. Your uterus, meanwhile, prepares itself for possible fertilization by thickening its lining (called the endometrium) so that the fertilized egg will implant have a place to stay and get nourishment while it grows to become a baby. If fertilization doesn't occur, then the lining prepared by the uterus must be shed and that is what comes out as your menstruation.
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