In a typical menstrual cycle, estrogen made by the maturing ovarian follicle causes the glands to grow and the endometrium to thicken (partly through an increased blood supply).This thickening of the uterine lining is called the proliferative phase of the menstrual cycle.n a typical menstrual cycle, estrogen made by the maturing ovarian follicle causes the glands to grow and the endometrium to thicken (partly through an increased blood supply).