Women of childbearing age usually about a month at a time from the last menstrual period, the first day of menstruation date to the day before the menstruation, called a menstrual cycle.
Female menstruation is about 13~14 years old, because of ovarian function in early puberty is in an unstable state, the menstrual cycle is usually inaccurate. But with the gradual development of female ovarian maturity, menstrual cycle will gradually tend to be accurate, regular. Usually women 45 to 55 years old, ovarian of pituitary gonadotropin and follicle reaction decreased, stop development, reduce the secretion of estrogen and progesterone, to stop endometrial cyclical changes, so that no menstruation, menopause.
The usual menstrual cycle is about 28 days, but “ the usual menstrual cycle ” is a relative concept, menstrual advance or lag for several days are normal.