Ovulation depends on hormones produced in two neighbouring regions located at the base of the brain known as the hypothalamus and the pituitary gland. Gonadotrophin releasing hormone (GnRH) produced by the hypothalamus acts on the pituitary causing it to release the hormone, FSH. FSH travels in the blood stream to the ovary and causes the development of follicles (which contain eggs) within the ovary. These growing follicles produce oestrogen. Rising oestrogen levels from ovarian follicles then cause the pituitary to release a surge of the hormone, LH, which acts on the lead follicle to bring about the release of its egg (ovulation). For more information on ovarian follicles, see my section on the Menstrual Cycle.