<unk>  The<unk>  is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs and the cyclic release of estrogen and progesterone. The uterine cycle governs the preparation and maintenance of the lining of the uterus (womb) to receive a fertilized egg. These cycles are concurrent and coordinated, normally last between 21 and 35 days in adult women, with a median length of 28 days, and continue for about 30–45 years. Naturally occurring hormones drive the cycles; the cyclical rise and fall of the follicle stimulating hormone prompts the production and growth of oocytes (immature egg cells). The hormone estrogen stimulates the uterus lining to thicken to accommodate an embryo should fertilization occur. The blood supply of the thickened lining (endometrium) provides nutrients to a successfully implanted embryo. If implantation does not occur, the lining breaks down and blood is released. Triggered by falling progesterone levels, menstruation (a "period", in common parlance) is the cyclical shedding of the lining, and is a sign that pregnancy has not occurred. Each cycle occurs in phases based on events in the ovary (ovarian cycle) or the uterus (uterine cycle). The ovarian cycle consists of the follicular phase, ovulation, and the luteal phase; the uterine cycle consists of the menstrual, proliferative and secretory phases. Day one of the<unk>  is the first day of the period, which lasts for about five days. Around day fourteen, an egg is usually released from the ovary. Menarche (the onset of the first period) usually occurs around the age of twelve years. The<unk>  can cause some women to experience problems that disrupt daily lives. These can include cramps, tender breasts, tiredness, and premenstrual syndrome. More severe problems such as premenstrual dysphoric disorder are experienced by 3–8% of women. The<unk>  can be modified by hormonal birth control.
Menstrual cycle
Metabolic age
Sex after pregnancy
Bone age
Sleep sex
Anovulatory cycle
Menstruation
Menstrual leave