Estrogen or oestrogen is a group of female hormones. It includes oestrone (E1), oestradiol (E2), and oestriol (E3). Oestradiol is the most important oestrogen. Oestrogen is mainly secreted by the ovary, a small amount by the liver, adrenal cortex, and breast. In pregnancy, the placenta can also have a lot of secretion. Male testes also secrete a small amount. Ovary mainly secretes β-estradiol, other estrogens are less important.