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The menstrual cycle is the regular natural change that occurs in the uterus and ovaries that make pregnancy possible. The cycle is required for the production of ovocytes, and for the preparation of the uterus for pregnancy.〔 Up to 80% of women report having some symptoms during the one to two weeks 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%, they are severe. The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be 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 is governed by hormonal changes.〔 These changes can be altered by using hormonal birth control to prevent pregnancy. Each cycle can be divided into three phases based on events in the ovary (ovarian cycle) or in the uterus (uterine cycle).〔 The ovarian cycle consists of the follicular phase, ovulation, and luteal phase whereas the uterine cycle is divided into menstruation, proliferative phase, and secretory phase. Stimulated by gradually increasing amounts of estrogen in the follicular phase, discharges of blood (menses) flow stop, and the lining of the uterus thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two become dominant (non-dominant follicles shrink and die). Approximately mid-cycle, 24–36 hours after the luteinizing hormone (LH) surges, the dominant follicle releases an ovocyte, in an event called ovulation. After ovulation, the ovocyte only lives for 24 hours or less without fertilization while the remains of the dominant follicle in the ovary become a corpus luteum; this body has a primary function of producing large amounts of progesterone. Under the influence of progesterone, the uterine lining changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within approximately two weeks, the corpus luteum will involute, causing a sharp drops in levels of both progesterone and estrogen. The hormone drop causes the uterus to shed its lining in a process termed menstruation. Menstruation also occurs in some other animals including shrews, bats, and other primates such as apes and monkeys. == Onset and frequency == The average age of menarche is 12–15.〔 They may occasionally start as early as eight, and this onset may still be normal.〔 This first period often occurs later in the developing world than the developed world.〔 The average age of menarche is about 12.5 years in the United States, 12.7 in Canada, 12.9 in the UK and 13.1 years in Iceland. Factors such as genetics, diet and overall health can affect timing.〔(【引用サイトリンク】 At what age does a girl get her first period? )〕 The cessation of menstrual cycles at the end of a woman's reproductive period is termed menopause. The average age of menopause in women is 52 years, with anywhere between 45 and 55 being common. Menopause before age 45 is considered ''premature'' in industrialised countries.〔(【引用サイトリンク】 work = NHS )〕 Like the age of menarche, the age of menopause is largely a result of cultural and biological factors; however, illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier than it might have otherwise. The length of a woman's menstrual cycle typically varies somewhat, with some shorter cycles and some longer cycles. A woman who experiences variations of less than eight days between her longest cycles and shortest cycles is considered to have regular menstrual cycles. It is unusual for a woman to experience cycle length variations of less than four days. Length variation between eight and 20 days is considered as moderately irregular cycles. Variation of 21 days or more between a woman's shortest and longest cycle lengths is considered very irregular. The average menstrual cycle lasts 28 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 35 to 39. Subsequently, the variability increases slightly for women aged 40 to 44.〔 Usually, length variation between eight and 20 days in a woman is considered as moderately irregular menstrual cycles.〔 Variation of 21 days or more is considered very irregular.〔 The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14/13 days, SD 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days). The follicular phase also seems to get significantly shorter with age (geometric mean 14.2 days in women aged 18–24 vs. 10.4 days in women aged 40–44).〔 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「Menstrual cycle」の詳細全文を読む スポンサード リンク
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