The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purposes of sexual reproduction and fertilization. This article focuses on the human menstrual cycle, a "monthly" cycle that takes approximately 28 days to complete.
The menstrual cycle, under the control of the endocrine system, is necessary for reproduction. It is commonly divided into three phases: the follicular phase, ovulation, and the luteal phase. It is also occasionally misclassified using the uterine cycle: menstruation, proliferative phase, and secretory phase. Menstrual cycles are counted from the first day of menstrual bleeding. Hormonal contraception interferes with the normal hormonal changes with the aim of preventing reproduction.
Stimulated by gradually increasing amounts of estrogen in the follicular phase, discharges of blood (menses) slow then 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 atrophy and die). Approximately mid-cycle, 24–36 hours after the Luteinizing Hormone (LH) surges, the dominant follicle releases an ovum, or egg in an event called ovulation. After ovulation, the egg 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 endometrium (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 sharp drops in levels of both progesterone and estrogen. These hormone drops cause the uterus to shed its lining and egg in a process termed menstruation.
In the menstrual cycle, changes occur in the female reproductive system as well as other systems (which lead to breast tenderness or mood changes, for example). A woman's first menstruation is termed menarche, and occurs typically around age 12-13. The average age of menarche is about 12.5 years in the United States, 12.72 in Canada, 12.9 in the UK and 13.06 ± 0.10 years in Iceland. The end of a woman's reproductive phase is called the menopause, which commonly occurs somewhere between the ages of 45 and 55.
Read more about Menstrual Cycle: Overview, Phases, Behavioral Change, Length, Fertile Window, Effect On Other Systems, Cycle Abnormalities and Disorders
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Famous quotes containing the words cycle and/or menstrual:
“Oh, life is a glorious cycle of song,
A medley of extemporanea;
And love is a thing that can never go wrong;
And I am Marie of Roumania.”
—Dorothy Parker (1893–1967)
“Virginity is now a mere preamble or waiting room to be got out of as soon as possible; it is without significance. Old age is similarly a waiting room, where you go after life’s over and wait for cancer or a stroke. The years before and after the menstrual years are vestigial: the only meaningful condition left to women is that of fruitfulness.”
—Ursula K. Le Guin (b. 1929)