The menstrual cycle is the female reproductive cycle that makes eggs available for fertilization. Menstruation involves the hypothalamus, pituitary and ovary, and in humans lasts roughly 28 days (this can vary greatly between individuals).
The menstrual cycle can be divided into approximately two halves:
- the follicular phase. In the follicular phase, the biochemical and physiological events (secretions of LH, FSH and estrogen) support the growth of the follicle (the cell complex that surrounds and nurtures the egg) in the ovary and build up the lining of the uterus to receive a fertilized egg. At mid-cycle, when the egg is ready, a chemical signal -- a surge of LH and FSH -- tells the ovary to release the egg. This stage is called ovulation, and is the optimum time for fertilization to take place.
- the luteal phase. In the luteal phase, the remnants of the follicle (corpus luteum) keep secreting estrogen and progesterone to maintain the readiness of the uterus. If the egg is fertilized, then the luteal phase continues throughout pregnancy. If the egg is not fertilized, then the corpus luteum dies, the uterus sheds its lining, menses begins and another menstrual cycle ensues.
See the next pages for a detailed look at the events that occur during the menstrual cycle.