Why do babies spit milk?

Don't worry. It is normal for young babies to spit up regularly. The peak age for spitting up is 4 months.
 

How to calm a crying baby

See all videos

When your baby takes in air along with his breast milk or formula, the air gets trapped in with the liquid. The air has to come up, and when it does, some of the liquid comes up too, through his mouth or nose.

Some babies take in too much milk for their sizes, so sometimes they become overfilled and milk overflow through spitting up.

Another reason is a newborn's digestive system isn't fully developed. The muscles at the bottom of your baby's esophagus, which control whether food is coming or going, is yet to be fully developed. So spitting up is not necessarily a worry.

Source: http://www.babycenter.com/0_why-babies-spit-up_1765.bc