Anemia is characterized by an abnormal decrease in the amount of red blood cells. The amount of hemoglobin is clinically inferred from the hemoglobin concentration, and the severity of anemia is judged by the degree of abnormally low hemoglobin concentration. Anemia is caused by impaired erythropoiesis, blood loss or excessive destruction of red blood cells. 1.Red blood cell production reduction: such as congenital pure red blood cell aplastic anemia, intrauterine infection, deficiency nutritional defects such as iron and folic acid, and congenital leukemia. 2, blood loss: including pre-birth, at birth, post-birth bleeding, as well as medically induced blood loss. (1) Blood loss before birth: such as fetal-mother transfusion, fetal-fetal transfusion, fetal-placental transfusion. (2) Blood loss at birth: such as placenta praevia, placental malformation, umbilical cord malformation, etc.; birth injury intracranial hemorrhage, subcapsular tendon hemorrhage, liver and spleen rupture, etc. (3) Post-birth hemorrhage: including hemorrhage caused by coagulation factor deficiency and thrombocytopenia; hemorrhage caused by poorly ligated umbilical cord or re-opening of umbilical cord stump; gastrointestinal hemorrhage caused by stress ulcer, congenital gastric rupture, hematogenous blood loss, etc. 3. Excessive destruction of red blood cells: (1) Immune hemolysis: such as Rh or ABO hemolytic disease, drug-related hemolytic anemia, etc. (2) Infection: such as bacterial or TORCH infection. (3) Vitamin E deficiency: Vitamin E is important to maintain the integrity of the red blood cell membrane. (4) Premature anemia: Premature infants experience a drop in hemoglobin in the first few weeks after birth, and the lower the birth weight, the earlier the anemia appears and the more severe it is (it can drop to the lowest level of 70-90g/L in 4 – 8 weeks after birth), and the longer it lasts. The causes are: (1) short life span of erythrocytes; (2) rapid weight gain and hemodilution; (3) relatively high medical blood loss; (4) low congenital iron reserves, vitamin E deficiency, etc.; (5) low serum erythropoietin levels.