Final answer:
The proliferative phase begins when the uterine wall starts to thicken during the reproductive cycle. During this phase, the lining of the uterus grows, or proliferates, due to the secretion of estrogen by cells of the maturing ovarian follicle.
Step-by-step explanation:
The phase that begins when the uterine wall begins to thicken during the reproductive cycle is called the proliferative phase. During this phase, the lining of the uterus grows, or proliferates, due to the secretion of estrogen by cells of the maturing ovarian follicle. This phase also stimulates the cervix to secrete larger amounts of thinner mucus, which can help sperm swim through the cervix and into the uterus, increasing the chances of fertilization.