Gastroesophageal reflux happens when stomach contents come back up into the esophagus (the tube that connects the mouth and stomach). Reflux is very common in babies, who often spit up after they are fed. Babies usually outgrow reflux, and it’s less common among older children.

Reflux occurs when the sphincter (circular muscle) at the lower end of the esophagus relaxes. This muscle normally opens during swallowing to let food into the stomach and then closes to keep it there. With reflux, the muscle relaxes at the wrong time, allowing stomach contents to travel back up the esophagus.

In babies, this muscle usually gets stronger and remains closed better as they grow, and their reflux improves. For some children, however, the reflux gets worse and can become gastroesophageal reflux disease (GERD). If GERD isn’t treated, it can eventually lead to more serious health problems.