If the muscle between the esophagus and the stomach relaxes when the stomach is full, food might flow up the baby's esophagus. The muscle is called the lower esophageal sphincter.
Spitting up is common in babies. During their first three months, about half of all babies do it. The food backs up from the baby's stomach through the same tube that carries food to the stomach, called the esophagus. This condition is called gastroesophageal reflux, infant reflux or infant acid reflux.
A muscle between the esophagus and the stomach keeps food in the stomach. The muscle is called the lower esophageal sphincter. Until this muscle has time to mature, your baby might spit up. This happens mainly when the baby's stomach is full.