In the female reproductive system, the menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive age females of several mammals, including human beings and other apes. Humans are one of the few species that has a menstrual cycle with concealed ovulation. Most other placental mammals experience an estrus, or heat, where imminent ovulation is signaled to the males of the species. This article concentrates on the menstrual cycle as it occurs in human beings.
The menstrual cycle is under the control of the hormone system and is necessary for reproduction. Menstrual cycles are counted from the first day of menstrual flow, because the onset of menstruation corresponds closely with the hormonal cycle. The menstrual cycle may be divided into several phases, and the length of each phase varies from woman to woman and cycle to cycle. Average values are shown below:
| Name of phase | Days |
| menstrual phase | 1–4 |
| follicular phase (also known as proliferative phase) | 5–13 |
| ovulation (not a phase, but an event dividing phases) | 14 |
| luteal phase (also known as secretory phase) | 15–26 |
| ischemic phase (some sources group this with secretory phase) | 27–28 |
During the follicular phase the lining of the uterus thickens, stimulated by gradually increasing amounts of estrogen. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two follicles become dominant (non-dominant follicles atrophy and die). The dominant follicle releases an ovum or egg in an event called ovulation. (An egg that is fertilized by a spermatozoon will become a zygote, taking one to two weeks to travel down the fallopian tubes to the uterus. If the egg is not fertilized within about a day of ovulation, it will die and be absorbed by the woman's body. ) 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.
