Pyloric stenosis is a condition in which a baby's pylorus (the muscle between the stomach and the first part of the small intestine) gradually swells and thickens, which interferes with food entering the intestine. This causes the baby to vomit most or all of their feedings.

Vomiting caused by pyloric stenosis usually starts gradually and gets worse over time. As the pylorus becomes tighter, the baby vomits more frequently and more forcefully (projectile vomiting). As vomiting continues, the baby will lose weight and become dehydrated.

Pyloric stenosis can occur any time between birth and 5 months of age, but it most commonly develops about 3 weeks after birth. The start of symptoms may be delayed if the baby was premature.

Pyloric stenosis is corrected with surgery (pyloromyotomy).