The most common spit-up problem in babyhood is gastroesophageal reflux. This happens when the junction between the stomach and the esophagus isn’t as constricted or angled as it should be, and some of the stomach’s contents wash back up into the esophagus. While most babies spit up a little when they burp after a feeding, in babies with reflux, the quantity and frequency can be significantly greater.
Now, you can usually get some signs from your child that there’s a problem, depending on the age. Babies may arch their backs, trying to “get away” from the acid. They may also wheeze, cough, or have a hoarse cry, as acid is irritating to the throat and the voice box. Thankfully, kids usually outgrow reflux by the time they’re eight or nine months old, when they are spending more time upright and the esophageal sphincter tightens up. In babies, there’s no cause for concern or treatment unless she is in distress or not gaining weight. A lot of times, a baby’s spit-up may look like the whole meal of milk he just ate, but it’s really only a fraction.
Answer excerpted from You Raising Your Child