Spitting up is the gentle sloshing of stomach contents up and out of the mouth in babies, sometimes accompanied by a burp. It is different than vomiting.
Most babies spit up milk out of their mouths or noses. This is because the sphincter at the top of the stomach is often loose. In otherwise healthy, happy babies who are growing well, the spit-up is mostly milk, rather than stomach acid, and nothing needs to be done.
Babies gaining at least 6 ounces a week and with wet diapers at least every 6 hours are usually growing well. On average, spitting up peaks at 4 months and is over by about 7 months of age, though it can take longer.
Spitting up may be caused by a number of different things. The muscle at the top of the stomach may be too loose (so it can't hold things in), the valve at the bottom of the stomach may be too tight (so the stomach gets too full), or big air bubbles may take up too much room. Sometimes babies just drink too much, too fast. Spitting up is not likely to be due to a formula intolerance or allergy to something in a nursing mother's diet.
To reduce spitting up, burp your baby several times during and after feeding. Sit the baby upright, with your hand supporting the head. Let the baby lean over slightly, bending at the waist. The upright posture moves air to the top of the stomach, and the forward lean puts a little pressure on the stomach to eject the air, helping the baby to burp. Avoid pressure on the abdomen, which occurs when holding your baby over your shoulder -- this just causes more expulsion of stomach contents.
Here are other ways to reduce spitting up:
If "projectile" spit-up shoots across the room, it's important to see a doctor to rule out a condition called pyloric stenosis. Here, the too-tight valve at the bottom of the stomach needs to be treated.