Reflux occurs because a muscle at the top of the stomach, called the sphincter, is not fully developed. This may allow stomach contents to travel backwards up the oesophagus (food pipe). Spitting up usually does not cause discomfort. Your baby may not even seem not to notice or be bothered by it. Reflux is not the same as vomiting. Vomiting is forceful, usually brings up more than a tablespoon or 2 of liquid, and upsets the baby. Vomiting can be a sign of illness. Reflux is not the same as drooling and blowing bubbles, which is especially common once babies start teething.