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Menstruation, also known as a period or monthly, is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. Up to 80% of women report having some symptoms prior to menstruation.〔 Common symptoms include acne, tender breasts, bloating, feeling tired, irritability, and mood changes. These symptoms interfere with normal life, and therefore qualify as premenstrual syndrome, in 20 to 30% of women. In 3 to 8%, symptoms are severe. The first period usually begins between twelve and fifteen years of age, a point in time known as menarche.〔 However, periods may occasionally start as young as eight years old and still be considered normal.〔 The average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 31 days in adults (an average of 28 days).〔 Menstruation stops occurring after menopause which usually occurs between 45 and 55 years of age. Bleeding usually lasts around 2 to 7 days.〔 The menstrual cycle occurs due to the rise and fall of hormones. This cycle results in the thickening of the lining of the uterus and production of an egg which is required for pregnancy. The egg is released around day fourteen in the cycle and the thickened lining of the uterus is to provide nutrients for a potential developing baby. If pregnancy does not occur, the lining is released in what is known as menstruation.〔 A number of problems with menstruation may occur. A lack of periods, known as amenorrhea, is when periods do not occur by age 15 or have not occurred in 90 days. Periods also stop during pregnancy and typically do not resume during the initial months of breastfeeding. Other problems include painful periods and abnormal bleeding such as bleeding between periods or heavy bleeding.〔 Menstruation in other animals occurs in primates, such as apes and monkeys, as well as bats and the elephant shrew. ==Onset and frequency== The first menstrual period occurs after the onset of pubertal growth, and is called menarche. The average age of menarche is 12 to 15.〔 However, it may start as early as eight.〔 The average age of the first period is generally later in the developing world and earlier in developed world.〔 The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 31 days in adults (an average of 28 days).〔 The average age of menarche has changed little in the United States since the 1950s.〔 Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 31 days in adults (an average of 28 days).〔〔 Perimenopause is when fertility in a female declines and menstruation occurs less regularly in the years leading up to the final menstrual period, when a female stops menstruating completely and is no longer fertile. The medical definition of menopause is one year without a period and typically occurs between 45 and 55 in Western countries.〔〔 During pregnancy and for some time after childbirth, menstruation does not occur; this state is known as amenorrhoea. If menstruation has not resumed, fertility is low during lactation. The average length of postpartum amenorrhoea is longer when certain breastfeeding practices are followed; this may be done intentionally as birth control. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「menstruation」の詳細全文を読む スポンサード リンク
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