The most serious cause of vomiting in infancy is an intestinal obstruction, either partial or complete. The most common cause is pyloric stenosis, which is a blockage in the intestines that prevents milk from getting through and back up the esophagus.

Pyloric stenosis is the narrowing of the lower end of the stomach, which is called the pylorus. While the condition is seldom apparent in the first week or two after birth, the muscle that circles the pylorus gradually grows thicker until it squeezes the end of the stomach like a rubber band. When the pylorus is only partially obstructed, the milk trickles through, and baby appears only to spit up. But toward the end of the first month, as the opening becomes narrower, the milk backs up in the stomach and the stubborn stomach tries with great force to push the milk through the narrowed opening. Some leaks through, but most comes back up forcefully as projectile vomiting. Baby may spray the milk a distance of two feet (sixty centimeters) across your lap. Whereas the normal spitter dribbles in a burp cloth on your shoulder, the projectile vomiter spews the contents a few feet away. Picture an overfilled water balloon with a knot tied loosely at both ends. You squeeze the balloon (the stomach contracts), and you keep squeezing until a knot loosens and squish, the water shoots out. This resembles what occurs in a baby with pyloric stenosis.

• Persistent projectile vomiting
• Weight loss or failure to gain weight
• Signs of dehydration: wrinkly skin, dry mouth, dry eyes, and decreasing number of wet diapers
• Stomach swollen like a big, tense balloon after feeding and deflated after vomiting

Some babies may normally experience projectile vomiting once or twice a day if overfed, underburped, or jostled too much. But persistent projectile vomiting accompanied by weight loss and dehydration needs immediate medical attention.

When phoning your doctor, have answers ready for the following questions:

• How did the vomiting start? Suddenly or gradually?
• What are the characteristics of the vomit? Is it clear, green, curdled, or sour? Is it spit-up or projectile?
• How often is your child vomiting?
• What amount of vomit is produced each time?
• Are there any other household members sick with similar signs?
• Does baby's abdomen hurt? Where, and how much? Is it tense, balloon like, soft, caved in?
• Does baby have signs of dehydration?
• Overall, how sick does your baby seem?
• Is baby's condition getting worse, better or staying the same?
• What treatment have you tried?

Following the diagnosis it is not unusual for baby to need a day or two of rehydration with fluids given intravenously in the hospital before surgery. The operation to relieve the pyloric obstruction takes about a half hour and is done through a small incision (often via Laparoscopy) in the upper abdomen. Improvement is immediate, and recovery time short.