<P> 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 35 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 . </P> <P> 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 . </P> <P> 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 drop 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 closely related primates (apes and monkeys). </P> <P> 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 . </P>

During the secretory phase of the menstral cycle