Estrogen, or oestrogen, is the primary female sex hormone. It is responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens in females that have estrogenic hormonal activity: estrone, estradiol, and estriol. In females, it helps develop and maintain both the reproductive system and female characteristics, such as breasts and pubic hair. Estrogen contributes to cognitive health, bone health, the function of the cardiovascular system, and other essential bodily processes.