What are the stages of the menstrual cycle?

The menstrual cycle begins with the onset of menstruation, then proceeds to the proliferative phase in which the uterine lining becomes thicker and the ovaries prepare to release an egg. Ovulation occurs midway through the cycle, and results in the release of an egg into the uterus. The uterine lining then continues to develop, until the onset of menstruation at the beginning of a new cycle.

Menstruation marks the beginning of the menstrual cycle, and results in the shedding of the uterine lining, called the endometrium. This lining consists of dead and living cells, blood, and glycogen that build up throughout the cycle. Menstrual bleeding usually lasts about 5 to 7 days, and may be light, moderate or heavy. The intensity of bleeding varies throughout each day of menstruation for most women. Estrogen and progesterone levels are at their lowest points during menstruation.

Once the old lining has been shed, increasing levels of estrogen and progesterone stimulate growth of a new endometrium. Also during this time, the pituitary gland increases secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate development of ovarian follicles. This stage of the menstrual cycle is therefore called the proliferative phase. While LH and FSH levels increase steadily into the midpoint, estrogen levels peak immediately before the midpoint and begin to decline. Progesterone levels continue to increase steadily.

The sudden increase in estrogen causes levels of luteinizing hormone and follicle stimulating hormone to rise dramatically, reaching a peak at about 14 days into the menstrual cycle. This peak causes the ovarian follicle to burst at the surface of the ovary, releasing an egg into the fallopian tube – a process called ovulation. The egg then travels through the fallopian tubes to the uterus, where it dies within one day of release if not fertilized.

Once an egg has been released, the empty ovarian follicle becomes known as the corpus luteum, and begins to secrete estrogen and progesterone. These hormones cause development of the endometrium. Progesterone levels peak around day 21, causing the endometrium to reach a maximum thickness, enriched with blood vessels and glycogen. The ovarian follicle begins to regress, leading to a gradual decrease in estrogen and progesterone levels. Low levels of estrogen and progesterone then lead to the onset of menstruation, and the cycle begins again.