28-27
Uterine (Menstrual) Cycle and Menstruation
nThe menstrual  phase occurs approximately during days 1–5 of the cycle. This phase is marked by sloughing of the functional layer and lasts through the period of menstrual bleeding.
nThe proliferative phase follows, spanning approximately days 6–14. The initial development of the functional layer of the endometrium overlaps the time of follicle growth and estrogen secretion.
nThe last phase is the secretory phase, which occurs at approximately days 15–28. During the secretary phase, increased progesterone secretion from the corpus luteum results in increased vascularization and development of uterine glands.
nIf the oocyte is not fertilized, the corpus luteum degenerates, and the progesterone level drops dramatically.
nWithout progesterone, the functional layer lining sloughs off, and the next menstrual phase begins.