Babies don’t have big tummies. At birth, a baby’s tummy is about the size of a cherry. After 3 days, it has made it to walnut dimensions. What this means is that it really cannot hold very much. In fact, until she is about 4 months old, your baby’s stomach can hold only small amounts of milk at a time. This is one of the main reasons why you have to feed her so often early on. Too much milk during feedings can cause your baby to spit up or be fussy because she feels overly full. Spit-up is generally milk coming straight back up due to an overflow of that tiny tummy, or air bubbles swallowed during feeding.
Many babies spit up regularly. It is not painful, and she might not even realize she has done it. If your baby is healthy and gaining weight, you really do not need to worry about it, as it is just part of the development process.