Pyloric stenosis is an uncommon condition in infants that blocks food from entering the small intestine.

Typically, a muscular valve between the stomach and small intestine holds food in the stomach until it is ready for the next stage in the digestive process. This valve is called the pylorus valve. In pyloric stenosis, the pylorus muscles thicken and become abnormally large, blocking food from reaching the small intestine.

Pyloric stenosis can lead to forceful vomiting, dehydration and weight loss. Babies with pyloric stenosis may seem to be hungry all the time.

Surgery cures pyloric stenosis.


Symptoms of pyloric stenosis usually appear within 3 to 5 weeks after birth. Pyloric stenosis is rare in babies older than 3 months.

Symptoms include:

  • Vomiting after feeding. The baby may vomit forcefully, ejecting breast milk or formula up to several feet away. This is known as projectile vomiting. Vomiting might be mild at first. But over time, it becomes more severe as the pylorus opening narrows. The vomit may sometimes contain blood.
  • Persistent hunger. Babies who have pyloric stenosis often want to eat soon after vomiting.
  • Stomach contractions. You may notice wavelike contractions that ripple across your baby's upper stomach area soon after feeding but before vomiting. These contractions are part of the digestive process known as peristalsis. The contractions are caused by stomach muscles trying to force food through the narrowed pylorus.
  • Dehydration. Your baby might cry without tears or become lethargic. You might find yourself changing fewer wet diapers or diapers that aren't as wet as you expect.
  • Changes in bowel movements. Since pyloric stenosis prevents food from reaching the intestines, babies with this condition might be constipated.
  • Weight problems. Pyloric stenosis can keep a baby from gaining weight, and sometimes can cause weight loss.

When to see a doctor

See your baby's doctor if your baby:

  • Projectile vomits after feeding.
  • Seems less active or unusually irritable.
  • Urinates much less frequently or has noticeably fewer bowel movements.
  • Isn't gaining weight or is losing weight.


The causes of pyloric stenosis are unknown, but genetic and environmental factors might play a role. Pyloric stenosis usually isn't present at birth and probably develops afterward.

Risk factors

Risk factors for pyloric stenosis include:

  • Sex. Pyloric stenosis is seen more often in boys, especially firstborn children, than in girls.
  • Race and ethnicity. Pyloric stenosis is more common in white and Hispanic children.
  • Premature birth. Pyloric stenosis is more common in babies born prematurely than in full-term babies.
  • Family history. Studies found higher rates of this disorder among certain families. Pyloric stenosis develops in about 20% of male descendants and 10% of female descendants of mothers who had the condition.
  • Smoking during pregnancy. This behavior can nearly double the risk of pyloric stenosis.
  • Early antibiotic use. Babies given certain antibiotics in the first weeks of life — erythromycin to treat whooping cough, for example — have an increased risk of pyloric stenosis. In addition, babies born to mothers who took certain antibiotics in late pregnancy may have an increased risk of pyloric stenosis.
  • Bottle-feeding. Some studies suggest that bottle-feeding rather than breastfeeding can increase the risk of pyloric stenosis. Most of the people who participated in these studies used formula rather than breast milk, so it isn't clear whether the increased risk is related to formula or the mechanism of bottle-feeding.


Pyloric stenosis can lead to:

  • Failure to grow and develop.
  • Dehydration. Frequent vomiting can cause dehydration and a mineral imbalance. These minerals are called electrolytes. Electrolytes help regulate many vital functions.
  • Stomach irritation. Repeated vomiting can irritate your baby's stomach and may cause mild bleeding.
  • Jaundice. Rarely, a substance secreted by the liver can build up, causing a yellowing of the skin and eyes. This substance is known as bilirubin.

Dec. 02, 2022
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  2. Olive AP, et al. Infantile hypertrophic pyloric stenosis. https://www.uptodate.com/contents/search. Accessed Oct. 19, 2020.
  3. Danko ME, et al. Current management of pyloric stenosis. Seminars in Pediatric Surgery. 2022; doi:10.1016/j.sempedsurg.2022.151145.
  4. Hypertrophic pyloric stenosis. Merck Manual Professional Version. https://www.merckmanuals.com/professional/pediatrics/gastrointestinal-disorders-in-neonates-and-infants/hypertrophic-pyloric-stenosis. Accessed Oct. 19, 2020.
  5. AskMayoExpert. Suspected pyloric stenosis (pediatric). Mayo Clinic; 2019.
  6. Pyloric stenosis. American Pediatric Surgical Association. https://eapsa.org/parents/learn-about-a-condition/f-o/hypertrophic-pyloric-stenosis/. Accessed Oct. 19, 2020.
  7. Kashyap PC (expert opinion). Mayo Clinic. Oct. 19, 2020.


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