In parallel with the ovarian cycle, some cyclical modifications of the uterine mucosa (uterine cycle or menstrual cycle) also occur. The uterine cycle may be divided into three phases: proliferative, secretory and menstrual.
The proliferative phase is part of the follicular phase of the ovarian cycle, lasting from approximately days 5-14 of the uterine cycle). During this phase, the pars functionalis (stratum functionale endometrii) regenerates following its disintegration during the desquamation phase.
The secretory phase is part of the luteal phase, lasting from approximately days 15-18 of the cycle. This phase begins about a day after ovulation. Proliferation of the endometrium ceases. Glycogen granules accumulate in the basal cytoplasm of the epithelia. The uterine stroma becomes oedematous. The stroma cells hypertrophy, and become predecidual cells. Towards the end of the secretory phase (the last part of the luteal phase) the spiral arteries contract. There is underperfusion or ischemia of the pars functionalis.
The desquamation or menstrual phase (early follicular phase) designates the monthly bleeding, which lasts for about 4 to 5 days. The spiral arteries of the endometrium relax. Blood flows into the ischemic pars functionalis, the capillary walls rupture, causing bleeding into the functionalis. The functionalis degenerates and is ejected together with non-coagulated arterial and venous blood. The basal portion (stratum basale endometrii) is preserved. This marks the start of the next cycle.
If pregnancy occurs, the blastocyst secretes HCG (humane chorionic gonadotropin). Due to this, the corpus luteum is preserved. It develops further into the corpus luteum graviditatis, which ensures a high level of progesterone. In the secretion phase, the endometrial stroma is transformed into the decidua (Formation of the placenta). Menstruation does not occur.