Sometime's it's hard to tell the difference between a newborn vomiting, and one that's merely spitting up. Here's a clear definition of what to look for, and how to treat both conditions.
The difference between the two is vital, since both are caused by different things. This means if you try to treat a spitting up problem with a vomiting solution, you'll get a whole heap of nothing.
Spitting up is most commonly seen in newborns 0-3 months, but a mild form of it can persist until the 10th or 12th month. It is defined as the easy flow of milk/formula out of the mouth, usually accompanied by a burp.
It is not serious and generally does not affect weight gain and growth.
Spitting up is, in fact, a mild form of reflux, having to do more with the acid balance in the stomach, than muscle contractions.
Even though spitting up is most common in newborns less than 3 months old, you may notice an increase in the amount of spit up during the 3-6 month age range.
This is may be because the muscles at the lower end of the esophagus have loosened up, allowing for more reflux.
If your infant's height, weight, and other developmental milestones are all on track, the problem should fix itself in time. In the meantime...
Newborn vomiting is when the abdominal muscles and diagphram contract vigorously while the stomach is relaxed. Most newborn vomiting is caused by viral infections, which doctor's refer to as gastroenteritis. (They'll be so impressed you know that!)
Vomiting at this young age is very dangerous, because of the risk of dehydration. For more information on how to handle newborn vomiting, read this article.

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