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The Menstruation Cycle in Women During the menstruation cycle, a womans body sheds the lining of the uterus called the endometrium, which is grown in order to prepare for a fertilized egg. This is called the menstrual period, or more commonly just a period. This bleeding will last from the early teen years sometimes as early as 8 or 9 years old until the woman experiences menopause, usually around the age of 50.
The menstruation cycle lasts from the first day of bleeding until the first day of the next cycle of bleeding. Menstruation cycles are controlled by a womans hormones. Each cycle is the result of the brain sending hormone signals to the ovaries and uterus, telling them to prepare for a pregnancy.
Two particular hormones, estrogen and progesterone, have the biggest role in the changes the uterus experiences during each cycle.
Estrogen causes the uterus to build up its lining called the endometrium. Progesterone in the body increases when the ovary releases an egg during ovulation, which is experienced during the middle of a menstruation cycle. Increased levels of progesterone keep the lining of the uterus thick and prepared for pregnancy.
When the egg is not fertilized, this causes a big drop in levels of progesterone and estrogen, which then causes the lining in the uterus to break downwhich begins the menstrual period. Women usually begin their cycle around the ages of 11 and 14, although in recent years this age has begun to lower. The menstruation cycle is usually irregular for the first few years, but will often even-out by the time a woman is in her 20s. Women usually begin to experience less frequent menstrual periods during their menstruation cycle by the time they are in their 40s, and will usually go through menopause when they reach the age of 50.
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From: (monthly bleed) http://irregularperiods.info/menstruation-cycle/