Answer: Almost half of young babies spit up regularly. The peak age for spitting up – also known as reflux – is 4 months. When your baby swallows 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.Babies take in a lot of nourishment in relation to their size, and some of them really like to eat, so sometimes they become overfilled and, well, overflow. A newborn's digestive system isn't fully developed, either.