What are the uterine changes during the menstrual cycle?

During the preovulatory period, the uterus goes through two phases: menstrual phase and proliferative phase. During menstruation, the endometrial lining of the uterus is shed along with blood and uterine secretions. Menses, beginning on day 1 of the following cycle, starts with vasoconstriction of the spiral arterioles, which causes ischemia and necrosis.

The average duration of menstrual flow is 4 days. The necrotic tissue releases vasodilator substances, causing vasodilatation. The necrotic walls of the spiral arterioles rupture, causing hemorrhage and shedding of cells over a period of 4-6 days. During the proliferative phase, estrogens stimulate mitosis of the stratum basale (endometrial proliferation), which regenerates the stratum functionale. The endometrium grows from 0.5 to 5 mm in depth.

Estrogens stimulate angiogenesis (neovascularization) in the stratum functionale as well as stimulate the growth of secretory glands. The blood vessels become the spiral arterioles that perfuse the stratum functionale. The glands contain glycogen but are nonsecretory at this time. In the postovulatory period, the uterus enters its secretory phase during which the endometrium is prepared for the possible implantation of the [fertilized ovum.

The endometrium during this secretory phase is hyperemic and has a “Swiss cheese” appearance. Progesterone halts endometrial mitosis but causes maturation and differentiation of the endometrium, including elongation and coiling of the mucous glands (which secrete a thick viscous fluid containing glycogen) and spiraling of the blood vessels.