The menstrual cycle is under the control of the hormone system and is necessary for reproduction. It may be divided into several phases: menstruation, the follicular phase, and the luteal phase. Ovulation defines the transition from the follicular phase to the luteal phase. Counted from the first day of menstrual flow, the length of each phase varies from woman to woman and cycle to cycle. The average cycles length is 28 days.

Stimulated by gradually increasing amounts of estrogen in the follicular phase, 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). The dominant follicle releases an ovum, or egg, in an event called ovulation. After ovulation, 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 die, causing sharp drops in levels of both progesterone and estrogen. These hormone drops cause the uterus to shed its lining in a process termed menstruation.

Terminology

A woman's first menstruation is termed menarche, and is one of the later stages of puberty in girls. The average age of menarche in humans is 12 years, but is normal anywhere between ages 8 and 16. Factors such as heredity, diet and overall health can accelerate or delay menarche. "At what age does a girl get her first period?," from Menstruation and the Menstrual Cycle, National Women's Health Information Center (accessed June 11, 2005) The cessation of menstrual cycles at the end of a woman's reproductive life is termed menopause. The average age of menopause in women is 51 years, with anywhere between 40 and 58 being common. Menopause before age 35 is considered premature. The age of menopause is largely a result of genetics; however, illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier.

The length of a woman's menstrual cycle will typically vary, with some shorter cycles and some longer cycles. A woman who experiences variations of less than eight days between her longest cycles and shortest cycles is considered to have regular menstrual cycles. It is unusual for a woman to experience cycle length variations of less than four days. Length variation between eight and 20 days is considered moderately irregular. Variation of 21 days or more between a woman's shortest and longest cycle lengths is considered very irregular (see cycle abnormalities).