Spitting up is common in healthy babies. During their first three months, about half of all babies experience their stomach contents coming back up into the esophagus, a condition known as gastroesophageal reflux, infant reflux or infant acid reflux.
Normally, a muscle between the esophagus and the stomach (lower esophageal sphincter) keeps stomach contents where they belong. Until this muscle has time to mature, spitting up might be an issue — especially if your baby is relatively full.