Pyloric stenosis is a narrowing of the stomach's outlet, which is called the pylorus.
This is due to over-development of the surrounding rings of muscle. Vomiting develops when the stomach outlet, squeezed by excessive growth of surrounding muscle, is narrowed enough to interfere with the stomach's ability to empty. Frequent, projectile vomiting is usually evident between two and eight weeks of age in most infants with this condition. Initial symptoms may be mild in some infants and can be confused with a milk allergy. The cause of pyloric stenosis is unknown.