Normally, food passes easily from the stomach into the first part of the small intestine through a valve called the pylorus. With pyloric stenosis, the muscles of the pylorus are thickened. This prevents the stomach from emptying into the small intestine.

The exact cause of the thickening is unknown. Genes may play a role, since children of parents who had pyloric stenosis are more likely to have this condition. Other risk factors include:

• Certain antibiotics
• Too much acid in the first part of the small intestine (duodenum)
• Certain diseases a baby is born with, such as diabetes

Pyloric stenosis occurs most often in infants younger than 6 months. It is more common in boys than in girls.

Vomiting is the first symptom in most children:

• Vomiting may occur after every feeding or only after some feedings.
• Vomiting usually starts around 3 weeks of age, but may start any time between 1 week and 5 months of age.
• Vomiting is forceful (projectile vomiting).
• The infant is hungry after vomiting and wants to feed again.

Other symptoms appear several weeks after birth and may include:

• Abdominal pain
• Burping
• Constant hunger
• Dehydration (gets worse as vomiting gets worse)
• Failure to gain weight or weight loss
• Wave-like motion of the abdomen shortly after feeding and just before vomiting occurs

The condition is often diagnosed before the baby is 6 months old.

A physical exam may reveal:

• Signs of dehydration, such as dry skin and mouth, less tearing when crying, and dry diapers
• Swollen belly
• Olive-shaped mass when feeling the upper belly, which is the abnormal pylorus

Ultrasound of the abdomen may be the first imaging test. Other tests may include:

• Barium x-ray -- reveals a swollen stomach and narrowed pylorus
• Blood tests -- often reveal an electrolyte imbalance

Treatment for pyloric stenosis involves surgery to widen the pylorus. The surgery is called pyloromyotomy.

If it is not safe to put the infant to sleep for surgery, an endoscope is used. This is a long, flexible tube with a camera and a tiny balloon at the end. The balloon is inflated to widen the pylorus.

In infants who cannot have surgery, tube feeding or medicine to relax the pylorus is tried.

Surgery usually relieves all symptoms. As soon as several hours after surgery, the infant can start small, frequent feedings.

If pyloric stenosis isn't treated, a baby won't get enough nutrition and fluid. The child can become underweight and dehydrated.

Contact your health care provider if your baby has symptoms of this condition.

Congenital hypertrophic pyloric stenosis; Infantile hypertrophic pyloric stenosis; Gastric outlet obstruction; Vomiting - pyloric stenosis

Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM. Pyloric stenosis and other congenital anomalies of the stomach. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 355.

Seifarth FG, Soldes OS. Congenital anomalies and surgical disorders of the stomach. In: Wyllie R, Hyams JS, Kay M, eds. Pediatric Gastrointestinal and Liver Disease. 6th ed. Philadelphia, PA: Elsevier; 2021:chap 25.

Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.