The blue back of the baby’s hand may be caused by the thin skin on the back of the hand, or it may be caused by diseases such as congenital heart disease, pneumonia, etc. The baby’s body functions are in a state of growth and development. The baby’s body functions are in the state of growth and development, the skin of the hand is relatively thin, the veins under the skin are more obvious, so the back of the hand can show the phenomenon of blue. With the growth of the age of the symptoms can be gradually alleviated. Babies are more sensitive to the stimulation of the external environment, so when the temperature of the external environment is low, babies can appear purple hands and feet and other symptoms. Children with congenital heart disease have cardiovascular malformations, which often cause bruising when blood circulation is impeded. Lung diseases, such as pneumonia, can result in impaired metabolism, such as bruising and difficulty breathing. When a child becomes ill, he or she should go to the hospital immediately.