Menstruation represents the cyclical physiological preparation for potential pregnancy, followed by removal of endometrial contents if pregnancy does not occur. Most women of reproductive age are familiar with menstruation. The average age of menarche in the United States is approximately 11.5 years. Most menstrual cycles are anovulatory in the first year following menarche and may remain irregularly ovulatory for up to 3 years (although women and providers should be aware that ovulation and/or pregnancy may occur). For the next three to four decades, most women will menstruate every 21–35 days (average 28 ± 7 days). Bleeding is variable but generally lasts 3–5 days (1–7 days may be considered normal) and is of variable intensity (but generally less than 3 oz or 90 cm³).

Although generally considered an ovarian and uterine phenomenon, the normal menstrual cycle may be considered as a comprehensive physiological adaptation in preparation for possible pregnancy. In addition to the uterine and ovarian changes described here, changes can be noted in the cervix, vagina, breast, and core body temperature. The cervical mucus becomes thinner with increased pH to facilitate entry of sperm. Vaginal epithelial cells also undergo change. Mammary ducts proliferate under estrogen and progesterone stimulation, which may lead to breast swelling and tenderness. A small spike in basal body temperature can be seen at the time of ovulation. This observation has contributed to the use of basal body monitoring in fertility management.

Physiologically, bleeding represents the end of one cycle. From the perspective of the patient and the provider, however, bleeding is the most easily identified aspect of the menstrual cycle and is, therefore, used to mark the beginning of each cycle. The first day of menstrual bleeding is day 1 with each day numbered sequentially through the last day prior to the reoccurrence of bleeding. Each menstrual cycle can be divided into two halves that differ in hormonal and physiological events. In a typical or average menstrual cycle, each half is approximately 14 days in duration.

The first half of each menstrual cycle is marked by endometrial proliferation and follicular development. In the first week of each menstrual cycle, multiple follicles enlarge. At approximately 1 week, a single follicle becomes dominant and the others involute, becoming atretic. The dominant follicle will, with appropriate hormonal regulation, continue to develop and will eventually rupture releasing an ovum for possible fertilization. With release of the ovum on day 14, the follicle undergoes a series of stereotypic changes filling with blood, granulose, and thecal cell proliferation and displacement of blood by luteal cells (corpus luteum). The luteal cells produce progesterone, which serves to stabilize the thickened endometrium through the second half of the menstrual cycle. The period of follicle development is referred to as the follicular phase. The period of luteal production of progesterone is referred to as the luteal phase.

Follicular development in the first half of each menstrual cycle is marked by follicular production of estrogen and endometrial proliferation in anticipation of possible implantation of a fertilized ovum. This generally occurs late in the first week and throughout the second week of the menstrual cycle. The first half of the menstrual cycle is, for this reason, sometimes referred to as the proliferative phase. With ovulation and luteal production of estrogen and progesterone, uterine glands become active, secreting clear fluid. This phase is referred to as the secretory phase. The endometrium will remain stable and secretory for as long as the progesterone stimulation continues.

If fertilization fails to occur, the corpus luteum will lose function beginning in the second half of the fourth week (corpus albicans). With the loss of hormonal support, endometrial thinning and localized necrosis lead to sloughing of the proliferative portion of the endometrial lining and the onset of menses. Until menopause, this cycle will repeat more or less regularly each month.

The hormonal changes just described relate to preparation for release of the ovum and subsequent fertilization by sperm. As noted, however, these menstrual changes may occur in the absence of ovulation. In addition, under normal physiological conditions, pregnancy requires the presence of functional sperm in sufficient quantity to ensure fertilization of the released ovum.