During the proliferative phase in the uterus, the wall of the endometrium begins to thicken. This phase of the uterus begins at the end of menstruation and lasts until ovulation, when the egg is ejected from the ovary. Follicle-stimulating hormone (FSH), secreted by the anterior pituitary gland in the brain, targets the ovaries and triggers the maturation process of up to 25 follicles. Each month, only one egg is brought to maturity and is ejected from the Graafian follicle. About 24 hours before ovulation, the pituitary gland releases a surge of a second hormone, luteinizing hormone (LH), which stimulates the release of the egg out of the ovary and into the Fallopian tube.
During the proliferative phase in the uterus, the hormone estrogen is released from the maturing Graafian follicles in the ovaries. Estrogen stimulates the proliferation of cells in the endometrium of the uterus. Estrogen also plays a role in regulating the release of FSH and LH from the pituitary gland. The increasing levels of estrogen in the bloodstream stimulate the secretion of FSH and LH from the anterior pituitary. The increased levels of FSH and LH in turn further increase estrogen secretion from the follicles in the ovaries.