Menstruation refers to the monthly discharge through the vagina of the blood and tissues that were laid down in the uterus in preparation for pregnancy.
The cyclic production of hormones that culminates in the release of a mature egg (ovum) is called the menstrual cycle, which begins during puberty and ends at menopause. The first menstrual cycle is called menarche.
Hormones that control the menstrual cycle are produced by the hypothalamus, pituitary gland, and ovaries. The beginning of a menstrual cycle is marked by the maturation of an egg in an ovary and preparation of the uterus (womb) to establish pregnancy. Menstruation occurs when pregnancy has not been achieved.
The menstrual cycle is divided into four phases and is, on average, 28 days long (21–45 days). The onset of menstruation, called a period, monthly, menses, or menstrual period, begins a new menstrual cycle and is considered day one. This first phase usually lasts five days. Menstruation occurs in response to drops in the level of the hormone progesterone. It is estimated that a woman will have 500 menstrual periods in her lifetime.
The second phase of the menstrual cycle is called the follicular or proliferative phase. The ovary, in response to increasing levels of follicle stimulating hormone, begins the egg maturation process. Although 10–20 eggs begin to develop within follicles of the ovaries, usually only one egg reaches maturity. Follicles are clusters of cells that encase a developing egg, hence the name "follicular phase." Developing follicles release the hormone estrogen that stimulates the lining of the uterus, called the endometrium, to grow (proliferate) in preparation to receive an embryo (an egg that has been fertilized and begun dividing) and establish pregnancy. This is why the second phase is also called the "proliferative phase." This phase usually lasts through day 13.
The ovulation phase occurs in response to a surge in luteinizing hormone and is marked by the release of a mature egg from the follicle. Ovulation usually occurs on day 14.
The fourth phase is called the luteal, secretory, premenstrual, or postovulatory phase, and usually lasts from days 15–28. During this phase, the empty follicle, now called the corpus luteum, releases the hormone progesterone which further prepares the uterus for implantation of an embryo. The endometrium thickens because of cell growth, changes in blood vessels and glands, and increases in fluid. If pregnancy does not occur, the fall in progesterone levels initiates the onset of a new menstrual cycle. However, if pregnancy does occur, progesterone levels remain high and the endometrium is not shed.
In the United States, menstruation typically begins at 12.8 years of age in Caucasian girls and 12.4 years of age for African American girls. Factors that help to dictate the age at which menarche occurs include race, mother's age at menarche, nutritional status, body fat, as well as climate and elevation. Studies have shown that a body fat level of 17% is necessary for menstruation to begin.
Women who live together or work in close proximity tend to find that their cycles begin to coincide. During the menstrual cycle, the body releases hormones called pheromones, which may signal surrounding women's cycles to begin.
Puberty signals the maturation of a young woman's reproductive hormones. As a girl reaches puberty, the pituitary gland in the brain starts to produce the hormones that signal the ovaries to begin functioning. The interaction between these hormones and the hormones estrogen and progesterone causes the lining of the uterus to swell and thicken in anticipation of a fertilized egg. If the egg is not fertilized, the lining is discharged through the vagina, resulting in menstrual bleeding.
Belinda Rowland, Rebecca J. Frey PhD, The Gale Group Inc., Gale, Detroit,