DefinitionSpitting up is the gentle sloshing of stomach contents up and out of the mouth in babies, sometimes accompanied by a burp. It is different than vomiting.InformationMost babies spit up milk out of their mouths or noses. This is because the sphincter at the top of the stomach is often loose. In otherwise healthy, happy babies who are growing well, the spit-up is mostly milk, rather than stomach acid, and nothing needs to be done.Babies gaining at least 6 ounces a week and with wet diapers at least every 6 hours are usually growing well. On average, spitting up peaks at 4 months and is over by about 7 months of age, though it can take longer.Spitting up may be caused by a number of different things. The muscle at the top of the stomach may be too loose (so it can't hold things in),