Describe the three phases of the uterine (menstrual) cycle, including the days each occurs and the hormonal influences driving each phase.

The uterine cycle has three phases: menstrual (days 1-5), proliferative (days 6-14), and secretory (days 15-28). Low estrogen and progesterone levels allow endometrial shedding in the menstrual phase, rising estrogens rebuild the endometrium in the proliferative phase, and progesterone from the corpus luteum prepares the secretory endometrium for implantation. If no pregnancy occurs, corpus luteum degeneration lowers progesterone, causing the functional layer to slough off around day 28 and the cycle to restart.

During the menstrual phase (days 1-5), ovarian hormone levels are at their lowest normal levels, while gonadotropins are beginning to rise. The thick functional layer of the endometrium detaches and is shed with bleeding for 3-5 days. By day 5, growing ovarian follicles start producing more estrogens. In the proliferative phase (days 6-14), rising blood estrogen levels cause the basal layer of the endometrium to generate a new functional layer. The new layer thickens, glands enlarge, and spiral arteries become more numerous. Estrogens also induce endometrial cells to synthesize progesterone receptors. Ovulation occurs at the end of this phase, around day 14, triggered by the LH surge. In the secretory phase (days 15-28), progesterone from the corpus luteum acts on the estrogen-primed endometrium. Spiral arteries elaborate, endometrial glands enlarge, coil, and secrete nutrients into the uterine cavity. Progesterone also makes cervical mucus viscous, forming a cervical plug. If fertilization does not occur, the corpus luteum degenerates near the end of this phase, progesterone levels fall, the spiral arteries kink and go into spasm, and the ischemic endometrial cells die. The final constriction and sudden relaxation of the spiral arteries causes the functional layer to slough off, and menstruation begins again on day 28.