What is Uterine or Endometrial Cycle?

The cycle of the uterine lining consists of three distinct phases:

• Menstrual
• Proliferative
• Secretory

Each phase is controlled by the hormones discussed in the previous section.

The Menstrual Phase

Because we count the beginning of the reproductive cycle from the first day of menstruation, this phase begins the endometrial cycle. Menstruation is the shedding of the lining of the uterus that had been built up during the previous two phases. As the ovarian corpus luteum slows and stops functioning, secretion of estrogen and progesterone decreases. In response to the dropping levels of these hormones, the blood vessels of the endometrium go into spasm, there is oxygen loss to the tissue (necrosis), and the lining sloughs off. Menstrual flow consists of blood, mucus, and cells from the lining of the uterus. No other organ in the body sheds and regenerates cyclically for years as does the uterus.

Menstruation usually lasts between 3 and 6 days, although it can be shorter or longer. Following menstruation, the proliferative phase of the endometrial cycle begins.

The Proliferative Phase

During this phase, under the influence of estrogen secreted by the ovarian follicle as it matures, the endometrium, which is thin following menstruation, begins to thicken. Cells lengthen, blood vessels reform in the endometrial lining, and glands grow or proliferate. The height of the endometrium grows from .5 millimeter to as much as 5.0 millimeters. This is important as the endometrial height can be measured via sonogram; and beyond 5.0 millimeters may indicate a problem known as endometrial hyperplasia. If not treated, over time this condition can lead to cancer of the endometrium. An endometrial biopsy in which a sample of the tissues of the endometrium is taken via a tiny suction tube provides a more definitive diagnosis. The proliferative phase is variable in length from woman to woman and from cycle to cycle in the same woman. It may last from 7 to 21 days.

The Secretory Phase

Once ovulation has occurred, progesterone becomes the dominant ovarian hormone as it is the main hormone secreted by the newly created corpus luteum-formed from the follicle through which the egg just ruptured. Under the influence of progesterone, the endometrium enters its secretory phase. This phase is consistently 12 to 16 days long. During this phase, the endometrium continues to thicken, blood vessels grow and open, and glands grow and secrete a fluid rich in glycogen (sugar). As many as fifteen thousand endometrial glands open onto the endometrial surface during the secretory phase of the cycle. The lining of the endometrium becomes more dense, but growth in height is inhibited.