The uterine cycle, as mentioned, coincides with the ovarian cycle and is divided into three phases: the menstrual phase, the proliferative phase and the secretory phase.

Menstrual phase: begins on the first day of menstruation and lasts three to five days. It is so called because it corresponds to the period of menstruation, the peeling of the walls of the uterus. At the end of the luteal phase of the ovarian cycle, the drop in estrogen and progesterone caused by the degeneration of the corpus luteum determines the reduction of blood flow to the outermost tissue that lines the uterus. As a result this tissue dies and begins to separate from the more stable support tissues below, which remain intact. Dead tissue gradually peels off, causing blood vessels to rupture and bleed. Over the next few days, a mixture of blood and peeling tissue flows from the uterus to the vagina and exits the body, a phenomenon called menstrual flow.

Proliferative phase: with the end of the menstrual flow begins the proliferative phase. The uterus renews itself in preparation for a possible pregnancy. The outermost tissues of the uterus begin to grow, the glands enlarge and the blood vessels increase the flow of blood. In the cervical canal, the glands begin to secrete a thin mucus that wets the inner surface; if the spermatozoa were deposited in the vagina, this mucus would facilitate their migration into the uterus, a fundamental step for fertilization. The uterine changes that occur at this stage are promoted by estrogen, whose plasma levels increase due to the growth and increased secretory activity of the dominant follicle.

Secretory phase: this phase coincides with the luteal phase of the ovarian cycle. During this phase, the endometrium (mucous membrane that lines the internal cavity of the uterus), which was rebuilt during the proliferative phase, is transformed in such a way as to make it a favorable environment for implantation and the subsequent reception and nourishment of the embryo in development. The blood supply to the endometrium becomes greater as the arteries branch. The glands of the endometrium enlarge and begin to secrete fluids rich in glycogen, which the embryo uses during its first stages of growth as a source of energy.

The fluid secreted by the glands of the cervix becomes denser, forming a real plug that prevents bacteria from entering the uterus from the vagina. These uterine changes are promoted by the hormonal action of the progesterone, whose blood levels increase during the secretory phase, thanks to the action of the corpus luteum. Towards the end of the secretory phase, if fertilization does not occur, the corpus luteum degenerates, causing the levels of estrogen and progesterone in the plasma to drop. This decline leads to a decrease in the influences of sex hormones on promoting the growth of the endometrium that generates the menstruation events described above. With the onset of menstruation, the secretory phase ends, and a new menstrual phase begins. If fertilization and implantation have taken place, the corpus luteum does not degenerate and estrogen and progesterone levels remain elevated. As a result, the characteristics of the uterine environment of the secretory phase are maintained during pregnancy.