The baby’s arm suddenly swollen, consider is due to insect bite dermatitis, muscle injury, hypoproteinemia and other factors. 1. insect bite dermatitis: if the baby’s arm is mosquito bites, will stimulate the local vascular permeability increase, thus triggering the tissue fluid exudation, so that the patient suddenly swollen arm situation. 2. Muscle injury: If the baby’s arm is hit, or sprained, it will lead to local muscle pulling, which will cause the local blood vessels and soft tissues to rupture, causing sudden swelling of the arm. 3. Hypoproteinemia: If the baby suffers from hypoproteinemia, the plasma albumin will be reduced, thus causing a decrease in osmotic pressure, resulting in plasma water and tissue extravasation, so that the baby suddenly swollen arms, usually symmetrical generalized edema. The baby’s arm suddenly swollen, there may be other reasons, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, give targeted treatment.