The menstrual cycle is the set of recurring physiological changes in a female's body that are under the control of the reproductive hormone system and necessary for reproduction. In women, menstrual cycles occur typically on a monthly basis between puberty and menopause. Besides humans, only other great apes exhibit menstrual cycles, in contrast to the estrus cycle of most mammalian species.
During the menstrual cycle, the sexually mature female body releases one egg (or occasionally two, which might result in dizygotic, or non-identical, twins) at the time of ovulation. The lining of the uterus, the endometrium, builds up in a synchronised fashion.
After ovulation, this lining changes to prepare for potential implantation of the fertilised egg to establish a pregnancy. If fertilisation and pregnancy do not ensue, the uterus sheds the lining and a new menstrual cycle begins. The process of the shedding of the lining is called menstruation.
Menstruation manifests itself to the outer world in the form of the menses (also menstruum): essentially part of the endometrium and blood products that pass out of the body through the vagina. Although this is commonly referred to as blood, it differs in composition from venous blood.
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