Estrogen or oestrogen is the primary female sex hormone. Estrogens are present in significant amounts in both men and women. They are present in higher amounts in women after menarche (onset of menstrual periods at puberty) until menopause. The primary function of estrogens is the development of female secondary sexual characteristics. These include the development of breasts, endometrium, regulation of the menstrual cycle, etc. In males, estrogen helps in the maturation of the sperm and the maintenance of a healthy libido. Estrogen helps in causing physical differentiation of the foetus.