Normally, the placenta begins to function after 12 weeks of pregnancy. Usually the placenta is formed in the 12th week of pregnancy, when it is almost completely grown. The placenta is an important organ between the fetus and the mother to maintain the growth and development of the fetus, with functions of defense, material exchange, synthesis and immunity. In early pregnancy, the progesterone required for fetal growth and development is mainly secreted by the corpus luteum of pregnancy, which is located in the ovary, and after 12 weeks of gestation, the placenta can take over its role to secrete progesterone. At the same time, the placenta can also produce various hormones such as progesterone and estrogen, as well as enzymes and cytokines, which play an important role in the maintenance of normal pregnancy. In addition, because the placenta is not fully grown in the first 3 months of pregnancy, the embryo is not stable in bed, so attention should be paid to avoid strain, emotional excitement and intercourse to avoid abnormalities such as miscarriage.