While you might think nine months is long enough for a baby to develop in the womb, he or she will not actually be fully developed upon arrival. Yes, all of the body parts will be present and accounted for, but some parts are not yet ready to perform their intended duties. For example, it will take a while for the brain to develop, for the teeth to show up, and for the muscles to build up. One of the more inconveniently slow body parts to develop is the digestive tract, specifically the valve at the base of the esophagus. Because this valve will not fully develop until your baby is approximately half a year old, spitting up will be a common occurrence. 

Spitting up is extremely common, and almost half of all babies spit up regularly. There is a good chance your baby will be spitting up until he or she is six months old or so, and most babies spit up a lot at around four months old. However, a few unlucky babies will not stop until they are a year old. Generally, when your baby learns to sit up alone is when the valve will strengthen.

Spit up is not a cause for concern, but make sure your baby is not vomiting. Vomit will be more forceful, and your baby will seem a lot more upset about it. If your baby seems to expel liquid all over you as if nothing happened, he’s just spitting up. Always call your doctor right away if you notice that your baby isn’t gaining weight or if there is any green discoloration in the vomit or spit up. Both of these symptoms could indicate a more serious condition.

To save yourself some laundry, there are ways you can prevent spit up occasionally. Feed your baby in an upright position so that the food doesn’t sit in his esophagus for too long. That way, it’ll go straight to his belly and be digested before it can come back up. You should also burp your baby often, and don’t get discouraged if he or she doesn’t do it right away. Keep trying, and give the air a chance to come up to the surface.

Spit up is inconvenient but completely normal. It’s gross at first, but you’ll certainly get used to it, and the daily laundry will simply become part of your routine.

Source: Zahid Kahn et al: Gastro Esophageal Reflux: An Over Investigated Entity In Neonates And Infants. Journal of Pakistan Medical Association Volume 60 Issue 12 December 2010