Heparin is given during pregnancy for two purposes: one is to protect the fetus; the other is to prevent venous thrombosis. In early pregnancy, there are some pregnant women whose blood inside the body belongs to the state of high coagulation and also have diseases with high blood coagulation status, in this case, if the coagulation is higher, it will easily lead to miscarriage, so they need heparin to preserve the fetus, like many who do embryo transfer. In late pregnancy, there are many pregnant women who are at high risk for thrombosis, such as high D-dimer, slow blood flow in the inferior vena cava, and accumulation of red blood cells. These women have a very high risk of developing venous thrombosis after delivery, and therefore require anticoagulation with heparin.