Menstrual cycles are controlled from beginning to end by hormones. Different female hormones are responsible for recruiting, maturing and releasing oocytes, or eggs, so they're ready to be fertilized by sperm. Four hormones are responsible for maturing an egg so it can be released from the ovary: gonadotropin-releasing hormone (GnRH); estradiol, a form of estrogen; follicle-stimulating hormone (FSH); and luteinizing hormone (LH).
GnRH Stimulates Follicle Stimulating Hormone
Egg stimulation begins with GnRH production by the hypothalamus, an area of the brain responsible for the release of several hormones. GnRH is released in short bursts known as a pulsatile pattern and stimulates pituitary production of LH and FSH.
FSH Stimulates Ovary
FSH levels rise slightly to stimulate a group of immature eggs to begin maturation in preparation for release. Between three and 30 eggs are chosen for recruitment, the Merck Manual states. Eggs stored in the ovary are present from birth, but don't finish their final maturation process until it's time for them to be released from the ovary, according to Florida Gulf Coast University.
Estradiol Matures Egg
The developing group of immature follicles, which are cysts that contain immature eggs, produces estradiol. Although a number of follicles start to mature, usually only one, called the dominant follicle, goes on to complete the full cycle to be released from the ovary. As the follicle matures, it produces more estradiol. The rise in estradiol inhibits the release of GnRH from the hypothalamus, so LH and FSH stop rising.
LH Stimulates Egg Release
LH is the hormone that home ovulation prediction kits test for, and also the hormone responsible for the final release of the egg from the ovary. LH levels remain fairly constant during the first part of the follicular phase of the menstrual cycle. When estradiol reaches a high level, LH is released from the pituitary in a surge that finishes the final maturation of the egg and moves the mature follicle away from the wall of the ovary. LH also signals the follicle to produce an enzyme that allows the egg to burst through the ovary wall. The ovulatory phase lasts 16 to 32 hours, from the rise in LH to the release of the egg from the ovary.