Virtually all babies have spit-up episodes at one time or another; the question is when and whether you should be worried about it. Most babies spit up because the muscular valve-like structure in their stomach does not seal up very tightly as the stomach churns. It's analogous to shaking a bag full of fluid without sealing off the bag — some spillage is inevitable.

However, if your baby has projectile vomiting — that is, the spit-up literally flies across the room — another condition might be causing the vomiting. This type of forceful vomiting is very different from the typical spit-ups, not only in its strength but in the consistency of the spit-up. Projectile vomiting may indicate the presence of pyloric stenosis, a condition in which the milk cannot travel past the stomach and into the intestine. It generally occurs in babies between the ages of two weeks and two months and is quite rare after six months of age. If your baby consistently spits up with great projectile force after feeding, your doctor needs to examine her for pyloric stenosis.

Furthermore, if there is any trace of blood in the spit-up or in the stool, you need to contact your doctor. This may be a sign of food allergy. Some babies are allergic to the protein in cow's milk, which is the type of protein in most commercial formulas. Consult your doctor about whether you need to switch to a different type of formula.