Days 5–14: Proliferative phase. In this phase, the endometrium rebuilds itself: Under the influence of rising blood levels of estrogens, the basal layer of the endometrium generates a new functional layer. As this new layer thickens, its glands enlarge and its spiral arteries increase in number. Consequently, the endometrium once again becomes velvety, thick, and well vascularized. During this phase, estrogens also induce the endometrial cells to synthesize progesterone receptors, readying them for interaction with progesterone. Normally, cervical mucus is thick and sticky, but rising estrogen levels cause it to thin and form channels that facilitate sperm passage into the uterus. Ovulation occurs at the end of the proliferative stage in response to the LH surge from the anterior
pituitary.