Which patients need to inject low molecular heparin: 1, pre-thrombotic state: pre-thrombotic state may not always occur thrombophilia, but may selectively affect the uteroplacental circulation and lead to microthrombosis of the placenta, resulting in a high incidence of miscarriage. 2.Antiphospholipid syndrome (APS): APS is a serious autoimmune disease, if left untreated about 70% or more will occur spontaneous abortion or intrauterine death. 3, slow rise of HCG: low molecular heparin has a promotional effect on the differentiation and invasion of trophoblast cells, promoting the growth of trophoblast cells and increasing HCG level. 4.Miscarriage caused by obesity, polycystic ovary syndrome, etc. is related to metaphase vascular thrombosis. The use of low molecular heparin in early pregnancy can inhibit thrombosis and significantly improve pregnancy outcome. 5.Patients with elevated uterine artery flow index in early pregnancy: low molecular heparin can reduce uterine artery blood flow resistance, improve uterine blood flow status and increase uterine blood flow, which has a positive effect on embryonic development. 6. Plasma protein C, protein S and antithrombin III deficiency: Defects in anticoagulant proteins can lead to imbalance in coagulation-anticoagulation mechanism or fibrinolytic activity, microthrombosis of uterine spiral arteries or chorionic vessels, resulting in poor placental perfusion or even infarction, and thus spontaneous abortion. 7. Repeated bed-holding disorders: injection of low molecular heparin before and after ovulation for subendometrial blood circulation, increasing endometrial tolerance, improving embryo implantation environment and increasing the bed-holding rate. 8. Other conditions: ovarian hyperstimulation syndrome, deep vein embolism, low amniotic fluid, intrauterine growth retardation, severe pre-eclampsia.