9-month-old baby sleeping on his back may be caused by physiological factors, colds, indigestion and other factors. 1. Physiological factors: the baby has been there since birth, the habit of sleeping on his back, may be a normal phenomenon, because this will make the baby feel secure, generally no other clinical manifestations, do not need to worry too much. 2. Colds: due to the baby’s body immunity is low, in the daily care is not appropriate, may be induced by colds and flu, causing nasal congestion, so that breathing is not smooth, in the lie on the stomach to sleep, can make the breathing a little more freely, may also be accompanied by sneezing, runny nose and other symptoms. 3. indigestion: the baby consumed cold food or abdominal cold, may lead to indigestion, abdominal pain, abdominal distension and other symptoms, the baby will sleep on his stomach, when sleeping on his stomach, can relieve abdominal discomfort. Nine-month-old baby sleeps on his back, and exclude physiological factors, there are other abnormal manifestations, parents are advised to take their children to the doctor for examination and treatment.