Estradiol is a kind of estrogen, which is mainly secreted by the ovaries in the early stage of pregnancy, and it is mainly secreted by the placenta after the formation of the placenta, and the secretion will increase, which mainly has the following effects: 1. Estradiol can promote the development of mammary ducts after pregnancy and make physiological preparation for breastfeeding after delivery, and it can also deepen the color of nipples and areolas and make Montserrat nodules appear. 2. Estradiol can also promote the uterus smooth muscle cells to thicken and increase the elasticity of the muscle layer, so that the uterus grows larger as the fetus develops.3. Estradiol can also change the nature of vaginal secretions after pregnancy, making the secretions sticky, which can stop the invasion of other pathogenic microorganisms and help protect the fetus in the uterine cavity.4. Estradiol can also increase the content of glycogen in the vaginal mucous membrane cells to maintain the pH value of the vagina and reduce vaginal inflammation during pregnancy.