Spitting up is when a baby spits out breast milk, formula, or other food during or shortly after feeding. It is different from vomiting, which is forceful; spitting up is effortless and does not seem to cause the baby any discomfort.
Some babies spit up for no reason. Other babies spit up because the valve at the top of the stomach does not close completely (reflux). Spitting up caused by reflux usually goes away as the baby gets older.
Frequent burping during feeding helps reduce the amount of spit up. Also, keeping the baby upright for 30 minutes after feeding can help.
Spitting up usually decreases when the baby is older than 4 months and starts eating solid food. Most babies stop spitting up by the age of 9 months.
WebMD Medical Reference from Healthwise