Baby’s left chest bulge may be related to vitamin D deficiency rickets, congenital rib deformity, pneumothorax and so on. 1. Vitamin D deficiency rickets: due to too little vitamin D in the baby’s body leads to an imbalance of calcium and phosphorus metabolism, so that calcium salts can not be deposited in the skeletal parts, resulting in deformities, such as chicken breast. 2. Congenital rib deformity: some babies have overgrown ribs and rib cartilage due to heredity or other factors, causing rib deformity, such as rib ectopia, which can also appear as a bulging of the left chest. 3. Pneumothorax: when the baby has pneumothorax, due to the gas filling in the chest cavity, it can lead to chest bulging, widening of the rib space, in addition, it can be accompanied by mediastinal emphysema, neck subcutaneous emphysema and other manifestations. Chest fluoroscopy can be perfected to confirm the diagnosis. The baby’s left chest bulging may have other causes, such as pneumothorax and other life-threatening, it is recommended to take the baby to the doctor in a timely manner, improve the examination, to clarify the cause of the disease and diagnosis, standardized treatment under the guidance of the physician.