There are many reasons why three-month-old babies get hiccups when they laugh, usually due to factors such as underdeveloped diaphragm, indigestion, improper feeding or care. 1. Underdeveloped diaphragm: Inhaling a lot of cold air when the child laughs can stimulate the phrenic nerve, resulting in sudden contraction of the diaphragm to spasm, so the hiccups will occur. 2. Indigestion: when the baby has indigestion, too much food accumulates in the stomach and can’t be emptied in time, and when the baby laughs, the pressure in the abdomen can be increased, which stimulates the phrenic nerve, thus causing hiccups. 3. Improper feeding or care: if the baby is cold, too fast to eat milk or cry before eating milk, the air will be swallowed into the stomach will also be hiccups. Three-month-old babies hiccups when they laugh, there may be other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.