4-month-old babies with blue on the back of the hand may be related to Mongolian spots, trauma, abnormal blood clotting function and other reasons. 1. Mongolian spots: the location of each baby’s birthmark may be different, some babies will grow on the back of the hand. The back of the hand blue is uneven, most of the unilateral hand, with age, some will subside on their own. 2. Trauma: 4-month-old baby skin is relatively delicate, may be the baby’s own play, accidentally touched a hard object, resulting in local capillary hemorrhage, the performance of bruising. 3. Abnormal coagulation function: if vitamin C deficiency or hereditary diseases, such as aplastic anemia. Due to poor coagulation function, blood can seep out from the capillary wall by itself, appearing the back of the hand blue. In addition to the above, 4-month-old babies with blue back of the hand can also be seen in other causes, parents are advised to bring their babies to the hospital in time, with the help of the doctor to clarify the cause, and actively cooperate with the doctor to deal with or treat.