Estradiol is an estrogen that is secreted by the follicles and corpus luteum in the ovaries and is important for the regulation of female characteristics and the menstrual cycle and sexual organs. For women during pregnancy, estradiol is produced by the ovaries and placenta and is one of the most important estrogens for women during pregnancy. Estradiol is one of the most important estrogens for women during pregnancy because it causes the enlargement of the breast ducts and the coloring of the nipples and areolas, as well as promoting the growth of the female breasts and preparing them in advance for breastfeeding of the baby after delivery. Therefore, during pregnancy, estradiol levels increase with the number of months of pregnancy. If estradiol is too low, it may be due to an endocrine disorder in the body and should be closely monitored for bleeding and abdominal pain. If there is a possibility of preterm miscarriage, go to the hospital to receive treatment to preserve the pregnancy if necessary.