Updated by: Arabinda Naik
This phase is so named because it is when the follicles in the ovary grow and form a mature egg. This phase includes a 6-14 days (about 2 weeks) cycle. Decrease in the level of progesterone and estrogen in the blood stimulates pituitary to release FSH (Follicular Stimulating Hormone) and LH which stimulate the ovarian follicle to secrete estrogen. It stimulates the growth, maintenance and functioning of secondary sex organs. Estrogen stimulates the proliferation (increase thickness) of the endometrium (2.3 mm) of the uterine wall more vascular and more glandular. FSH secreted under influence of GnRH (Gonadotropin releasing hormone) from the hypothalamus stimulates the conversion of a primary follicle into a graafian follicle. The graafian follicle then secretes estrogen. Increase in the level of estrogen stimulates the thickening of endometrium in the uterus. The endometrium becomes thicker by rapid increase of cells, blood vessels and endometrial glands. This thickening of the uterine wall is for implanted ovum if pregnant established. High levels of estrogen inhibit the secretion of LH and FSH.