One of a group of female steroid sex hormones, produced principally by the ovaries, that promote the onset of secondary sexual characteristics (such as breast enlargement and development in women) and control the oestrous cycle (menstrual cycle in humans). Oestradiol is the most important; others include estrone and estriol, which are less active than oestradiol. Oestrogens are secreted at particularly high levels during ovulation, stimulating the uterus to prepare for pregnancy. They are used in oral contraceptives (with progestogens) and as treatment for various disorders of the female reproductive organs. Small amounts of oestrogens are produced by the adrenal glands and testes. Oestrogens are synthesized ultimately from cholesterol via intermediates that include testosterone and other male sex hormones (see androgen). These intermediates are converted to oestrogens by the enzyme aromatase, which is found in the ovaries and also in adipose tissue, an important source of oestrogens in postmenopausal women.