1. Nervous system is not mature: due to the age of the baby, the nervous system is not mature, so the control of the body is relatively poor, like to make a hmmmm sound, but also inexplicable force.
2. Abdominal distension: if parents find that the baby has the above situation, accompanied by abdominal bulging, like a ball or a small watermelon shape, farting will also increase, may be caused by abdominal distension.
3. Rickets: Infants with vitamin D deficiency or calcium deficiency leading to rickets will also have the above condition.