Pyloric stenosis: symptoms, treatment, and when to worry
Last updated September 3, 2026.
Pyloric stenosis is when the muscular valve at the bottom of a baby's stomach thickens and blocks milk from passing into the bowel, causing forceful vomiting after feeds. It typically starts between 2 and 8 weeks of age, and it is fixed with a small, very safe operation. The key danger is dehydration, which is why the pattern should never be watched for long.
What does it look like?
The signature is projectile vomiting: milk that shoots forcefully out within about half an hour of a feed, in a baby who is immediately hungry again and eager to re-feed. Over days, the clues accumulate: fewer wet nappies, weight loss or failure to gain, a constantly ravenous baby, and sometimes visible waves rippling across the upper belly after feeding. It is commoner in first-born boys, and it does not settle on its own.
What actually helps?
- Same-day assessment for the pattern: projectile vomiting after every feed in a young baby is a today problem, not a routine one.
- Ultrasound confirms it: a quick, painless scan measures the thickened muscle and makes the diagnosis on the spot.
- Fluids before surgery: babies are rehydrated and their blood salts corrected with an IV drip first, because the vomiting disturbs body chemistry in ways that must be fixed before anaesthesia.
- The operation is small and definitive: a pyloromyotomy, splitting the thickened muscle, is done through keyhole or a tiny cut and cures the condition. Babies usually feed within hours and go home in a day or two.
- Recovery is typically complete: minor posseting can continue briefly, but the projectile vomiting stops, and there are no long-term feeding consequences.
When is it an emergency?
Get same-day urgent care for projectile vomiting after feeds, and emergency care now for dehydration signs: very few wet nappies, a sunken soft spot, dry mouth, no tears, unusual sleepiness or floppiness, or a baby too weak to feed. Green (bile-stained) vomit or blood in vomit in any baby is an emergency department visit immediately. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
How is pyloric stenosis different from normal baby spit-up?
Normal posseting is small, effortless dribbles in a content, thriving baby. Pyloric stenosis is forceful, projectile vomiting that empties the feed, after every feed, in a baby who is immediately hungry again and starts to lose or stop gaining weight. The pattern escalates over days. Reflux sits between the two: frequent vomiting but without the projectile force, hunger loop, and falling weight that mark the blockage.
What causes pyloric stenosis?
The muscle ring at the stomach's outlet, the pylorus, thickens after birth until it squeezes the channel shut. Why it happens in some babies is not fully understood; it runs in families somewhat and is commoner in boys and first-borns. It is not caused by anything about feeding technique, and it is nobody's fault. Nothing the parent does causes or prevents it.
How is pyloric stenosis diagnosed?
By ultrasound, which shows the thickened pyloric muscle directly and painlessly. Where the story is classic, sometimes a clinician can feel the thickened muscle as a small olive-shaped lump in the upper belly. Blood tests check the salt disturbances that prolonged vomiting causes, since those must be corrected with IV fluids before any anaesthetic.
What does the operation involve?
A pyloromyotomy: the surgeon splits the thickened muscle to open the channel, usually through keyhole surgery, in under an hour. Babies are rehydrated first, then typically feed within hours of waking and go home in a day or two. The cure is permanent, recurrence is essentially unheard of, and the long-term outcome is a completely normal eater.
Is pyloric stenosis an emergency?
It is urgent rather than blue-light: the same-day rule applies to the vomiting pattern, because dehydration and salt disturbances accumulate over days. It crosses into emergency territory when dehydration signs appear, few wet nappies, sunken soft spot, dry mouth, lethargy, or when vomit is green or blood-stained. Do not wait for a next-day appointment on those.
Can pyloric stenosis happen after 8 weeks?
The typical window is roughly 2 to 8 weeks, and onset beyond about 3 months is rare, though not impossible. Forceful vomiting starting in an older baby points the workup elsewhere, toward reflux complications, intolerance, or other surgical causes, which is one reason the age pattern matters to the clinician assessing a vomiting baby.