Hardness on one side of the baby’s chest may be caused by external impact, mammary gland development, inflammation or tumor. 1. External impact: If the baby’s chest is traumatized or impacted, a local hematoma will be formed, resulting in hardness on one side of the baby’s chest, and the hardness will disappear after the hematoma disappears. 2. Mammary gland development: various factors lead to precocious puberty, resulting in increased levels of estrogen and progesterone in the body, causing mammary gland development, which will cause the baby’s chest to be hard on one side. 3. Inflammation: If the chest and ribs are infected with inflammation, local enlargement will occur under the stimulation of inflammatory factors, resulting in hardness on one side of the baby’s chest. 4. Tumor: this situation occurs less often, common tumors are dermatoid cysts, breast fibroma, malignant tumors, etc., which can cause baby chest hardness on one side. If your baby’s chest is hard on one side, there may be other conditions, so you can go to the hospital to complete the relevant examinations to clarify the situation and take corresponding measures.