Though it may seem like your baby is spitting up a lot of milk, that's not likely to be the case. The liquid spit up during regurgitation is usually made up mostly of saliva and gastric juices, and only a small amount of milk. In older children and adults, an elastic-like muscle at the entry to the stomach closes like a valve to prevent liquids from being pushed back up. In babies, however, this valve or sphincter isn't fully effective until between 6 and 12 months of age. Since it isn't fully developed yet, the valve is easily pushed back by the contents of the stomach - resulting in regurgitation or spitting up.
Regurgitation often occurs after overfeeding, or in combination with burping.
Consequently, breastfed babies tend to spit up less than bottle-fed babies, because they usually take in only as much milk as they need, and because they tend to swallow less air. Regurgitation is not caused by allergy or food intolerance, and shouldn't be confused with vomiting. Unlike spitting up, vomiting is characterized by the forceful expulsion of the contents of the stomach. It's important to know the difference between vomiting and spitting up because repeated vomiting can be a signal of a more serious illness, and because it can easily lead to dehydration. Dehydration is a dangerous condition in which excessive loss of body fluids results in a potentially life-threatening imbalance of water and essential body salts.