The menstrual cycle is controlled by a series of hormones that are produced by various parts of the body: GnRH: A hormone that stimulates the production of gonadotropins (it is produced by the the hypothalamus that is in the brain). Gonadotropins: Follicular Stimulating Hormone (FSH) and Luteinising Hormone (LH) are produced by the pituitary gland, also located in the brain. Estrogens: Hormones produced especially by the ovaries. Progesterone: Also produced by the ovaries. GnRH produced by the hypothalamus stimulates the pituitary to release FSH. FSH travels through the bloodstream and finally stimulates the ovaries to produce oocytes. Approximately 15 to 20 oocytes that grow in microscopic cysts, known as follicles, begin to mature in the ovaries. One follicle (rarely 2 or more) develops faster than the others and is referred to as the leading follicle. Why this happens is not clear, but possibly some sort of 'natural selection' mechanism takes place. FSH also induces the ovaries to produce estrogens. These help the oocytes to further mature and also feed the endometrium (inner layer of the uterus) so that it is ready to support a potential pregnancy.