How does the ovarian cycle coordinate with the uterine cycle, and what hormonal events trigger ovulation?

The ovarian cycle and uterine cycle are coordinated by ovarian hormones. The follicular phase of the ovarian cycle overlaps the uterine menstrual and proliferative phases, while the luteal phase corresponds to the uterine secretory phase. Ovulation is triggered by a midcycle LH surge that occurs when high estrogen levels from the dominant follicle exert positive feedback on the anterior pituitary. This LH surge causes follicle rupture and formation of the corpus luteum.

In a 28-day cycle, the ovarian cycle’s follicular phase spans the uterine menstrual phase (days 1–5) and proliferative phase (days 6–14). During menstruation, low ovarian hormone levels allow the functional layer of the endometrium to shed. As FSH and LH stimulate follicle growth, the maturing follicles secrete estrogens, and rising estrogen levels rebuild the endometrium during the proliferative phase, causing the basal layer to generate a new functional layer with enlarged glands and more spiral arteries. Ovulation occurs at the end of the proliferative stage, around day 14. The luteal phase of the ovarian cycle then corresponds to the uterine secretory phase (days 15–28), during which progesterone from the corpus luteum acts on the estrogen-primed endometrium to make it secretory and receptive for implantation. The key hormonal trigger for ovulation is positive feedback: when estrogen levels from the dominant follicle reach a critical blood concentration, they briefly stimulate the anterior pituitary to release a sudden burst of LH (with some FSH). This LH surge causes the primary oocyte to complete meiosis I and promotes inflammatory events and enzyme release that weaken and rupture the ovarian wall, expelling the secondary oocyte. After ovulation, the LH surge also transforms the ruptured follicle into the corpus luteum, which secretes progesterone and estrogens. If pregnancy does not occur, the corpus luteum degenerates, ovarian hormone levels fall sharply, and the uterine cycle resets with menstruation.