The causes of high red blood cells in newborns mainly include physiological factors and pathological factors. 1. Physiological factors: newborns have not yet established independent respiration, so they are in a state of hypoxia, which will lead to a compensatory increase in red blood cells. There are also some newborns in high altitude areas will also be compensatory erythrocyte increase. 2. Pathologic factors: including relative erythrocytosis and absolute erythrocytosis. (1) Relative erythrocytosis: mainly due to diarrhea, profuse sweating, vomiting and some other reasons lead to a reduction in the body’s fluid content, the blood appears to be concentrated, and the concentration of red blood cells is also relative to the increase in phase. (2) Absolute erythrocytosis: It may be related to factors such as excessive placental input during pregnancy, medication taken by the mother and delayed tying of the umbilical cord. The specific cause of high red blood cells in newborns should be examined and diagnosed by a doctor for relative treatment.