All babies have reflux. Anatomically, there is a valve at the top of the stomach that is meant to close off and prevent backflow of stomach contents/acid up the oesophagus. In small babies this is never closed as it is not yet a mature muscle. Therefore, when babies are put in a lying down position, they get reflux. If babies take in too much milk and their stomachs become over inflated, their reflux symptoms worsen. This is why reflux starts to become an issue at about 3weeks of age, as volumes of milk are increasing.