Babies spit up primarily because their digestive systems are immature. The muscle at the top of their stomach (the lower esophageal sphincter) that keeps food in the stomach is not fully developed, so milk and stomach contents can flow back up into the esophagus and out of the mouth. Other factors that contribute include drinking too much milk too quickly, swallowing air while feeding, overfeeding, and sometimes having a forceful milk let-down from breastfeeding. Spitting up is very common and usually harmless if the baby is comfortable, growing, and feeding well. Most babies outgrow spit-up by around 12 months as their muscles mature. It is not typically caused by a formula intolerance or allergy. To reduce spitting up, caregivers can burp the baby regularly during and after feedings, feed smaller amounts more often, hold the baby upright after feeding, and avoid excessive movement immediately after feeding. In rare cases, excessive spit-up can indicate medical issues like gastroesophageal reflux disease (GERD) or pyloric stenosis, which require medical evaluation. In summary, babies spit up because their stomach muscles are immature, causing stomach contents to reflux back up, and this usually resolves naturally over time as the baby grows.