What are the phases of the uterine (menstrual) cycle and what happens in each?

The uterine (menstrual) cycle has three phases. The menstrual phase (days 1–5) sheds the functional layer of the endometrium with bleeding. The proliferative phase (days 6–14), driven by rising estrogens, rebuilds the functional layer. The secretory phase (days 15–28), driven by progesterone from the corpus luteum, prepares the endometrium for embryo implantation.

The uterine cycle is coordinated with the ovarian cycle and consists of three phases. During the menstrual phase (days 1–5), the thick, hormone-dependent functional layer of the endometrium detaches from the uterine wall and is shed along with blood as menstrual flow; this lasts 3–5 days. During the proliferative phase (days 6–14), rising estrogen levels from growing ovarian follicles cause the basal layer to generate a new functional layer, which thickens as its glands enlarge and spiral arteries increase in number. Ovulation occurs at the end of this phase, around day 14. During 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 to sustain a potential embryo before implantation. If fertilization has not occurred, the corpus luteum degenerates, progesterone levels fall, spiral arteries kink and go into spasm, and the functional layer dies and sloughs off, restarting the cycle on day 28.