The nursing instructor explains to a class that important effects of estrogen in the proliferative ...

ANS: B
During the proliferative phase of the uterine, or endometrial, cycle, increasing amounts of estrogen lead to changes in cervical mucus that facilitate sperm penetration at midcycle. During the ischemic phase, both estrogen and progesterone levels are low, and the uterine spiral arteries constrict, limiting blood flow to the endometrium. Eventually, the endometrial blood vessels rupture and menses begin. In the secretory phase, progesterone functions to create a highly vascular secretory endometrium that is suitable for implantation of a fertilized ovum.