Sex hormones are mainly secreted by the gonads of the reproductive system. In women, sex hormones are mainly secreted by the ovaries, while the adrenal cortex also secretes a certain amount of sex hormones, and the placenta of women after pregnancy also secretes sex hormones. The most important role of sex hormones is to promote the healthy development of the reproductive system and to maintain the shape and function of secondary sexual characteristics (such as breasts and vulva), and under the action of female estrogen and progesterone, to form the cyclic menstrual flow and the ovarian cycle of ovulation. The action of estrogen and progesterone causes the development of breast tissue in women. Sex hormones are also the main substances that stimulate sexual desire and enable sexual function. The female body also secretes small amounts of androgens, which have the important role of promoting protein synthesis, bone calcium deposition and bone growth.