Estrogen is a group of hormones primarily responsible for the development of female sex organs and secondary sex characteristics. While estrogen is one of the major female sex hormones, small amounts are found in males. In women, follicular stimulating hormone (FSH; produced by the pituitary gland) stimulates cells (follicles) surrounding the eggs in the ovaries, causing them to produce estrogen. When the estrogen levels reach a certain level, the pituitary produces a surge of luteinizing hormone (LH), which eventually causes the release of the egg, beginning the preparation for fertilization.