[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Digestive system](https://durgical.com/en/embryology/#a-digestive) › Hypertrophic pyloric stenosis [Embryology](https://durgical.com/en/embryology/) · [Digestive system](https://durgical.com/en/embryology/#a-digestive) # Hypertrophic pyloric stenosis - Malformation The pyloric muscle thickens in the first weeks of life and closes the stomach outlet: the baby vomits forcefully and is still hungry. Updated 2026-11-01 **See it in 3D:** [Stomach](https://durgical.com/en/anatomy/stomach/) · [Duodenum](https://durgical.com/en/anatomy/duodenum/) ## Key points - Typical onset: between the third and sixth week of life. - Projectile, non-bilious vomiting (the block is above the papilla), with an infant who is hungry afterwards. - An olive-shaped mass in the epigastrium and visible gastric peristaltic waves. - Hypochloremic, hypokalemic metabolic alkalosis from loss of gastric acid. - More common in boys (about 4 to 1) and in first-borns; linked to macrolides in the first weeks of life. - Ultrasound is the test of choice; treatment is fluid and electrolyte correction, then pyloromyotomy. ## What it is It is hypertrophy of the muscle layer of the pylorus that narrows the gastric outlet in young infants. It is not a failure of embryonic formation: the pylorus forms normally and its muscle overgrows after birth, for multifactorial reasons. It is the most common surgical cause of non-bilious vomiting in the first weeks of life and an exam classic for its picture and its metabolic disturbance. ## Embryological cause - The pylorus forms normally from the foregut; the disorder appears after birth. - The pyloric muscle fibres, mainly the circular ones, hypertrophy and lengthen the pyloric channel until it obstructs. - The cause is multifactorial and the exact mechanism is unknown: heredity, male sex, postnatal macrolide exposure, bottle feeding and maternal smoking all play a part. ## How it presents - Increasingly forceful, projectile, non-bilious vomiting after feeds from the third week of life. - A hungry infant who loses weight and becomes dehydrated. - A palpable pyloric olive in the epigastrium and peristaltic waves moving left to right. - Some infants have mild jaundice. ## Diagnosis - Ultrasound: thickened pyloric muscle and an elongated pyloric channel. - Blood gas and electrolytes: hypochloremic, hypokalemic metabolic alkalosis, now less frequent because diagnosis is earlier. - If a contrast study is done: the string sign through the narrowed pyloric channel. ## Memory aid An olive jammed in the stomach's doorway: the baby shoves hard (projectile), nothing green gets through (no bile), and the baby is left hungry. ## Test yourself Think of the answer, then tap the question to see it. ## Why is the vomit in pyloric stenosis free of bile? Because the obstruction is at the pylorus, before the bile duct opens into the duodenum. ## Which acid–base disturbance is typical? Hypochloremic, hypokalemic metabolic alkalosis, from loss of hydrochloric acid in the vomit. ## Which antibiotics are linked to a higher risk in newborns? Macrolides such as erythromycin given in the first weeks of life. ## What is the imaging test of choice? Abdominal ultrasound, which shows the thickened pyloric muscle and the elongated channel. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/hypertrophic-pyloric-stenosis/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Stomach](https://durgical.com/en/anatomy/stomach/) - [Duodenum](https://durgical.com/en/anatomy/duodenum/) ## Related - [Foregut: esophagus, stomach rotation and duodenum](https://durgical.com/en/embryology/foregut-esophagus-stomach-rotation-and-duodenum/) - [Duodenal atresia and stenosis](https://durgical.com/en/embryology/duodenal-atresia-and-stenosis/) - [Malrotation and midgut volvulus](https://durgical.com/en/embryology/malrotation-and-midgut-volvulus/) ## Related conditions - [Infantile hypertrophic pyloric stenosis](https://durgical.com/en/conditions/infantile-hypertrophic-pyloric-stenosis/) - [Metabolic alkalosis](https://durgical.com/en/conditions/metabolic-alkalosis/) - [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/) - [Dehydration (fluid volume deficit)](https://durgical.com/en/conditions/dehydration/) [← All of embryology](https://durgical.com/en/embryology/) Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given. Sources: [Pyloric Stenosis](https://www.ncbi.nlm.nih.gov/books/NBK555931/) (StatPearls Publishing (NCBI Bookshelf), 2023); [Pyloric stenosis in infants](https://medlineplus.gov/ency/article/000970.htm) (MedlinePlus, U.S. National Library of Medicine, 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/hypertrophic-pyloric-stenosis/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/hypertrophic-pyloric-stenosis/