The female hormone produced in the ovary. Responsible for formation of the female secondary sex characteristics such as large breasts; supports the growth of the follicle and the development of the uterine lining. At midcycle the peak oestrogen level triggers the release of the LH spike from the pituitary gland. The LH spike is necessary for the release of the ovum from the follicle. Fat cells in both obese men and women can also manufacture oestrogen from androgens and interfere with fertility.