The first time a baby projectile-vomits across the room, parents tend to look at each other and ask whether that was normal. It isn't — not in the way ordinary posseting is normal — and when it begins to happen consistently after feeds in a baby of around 2 to 8 weeks, pyloric stenosis is one of the things to think about.
The condition is important to recognise because it is progressive: a baby who is vomiting most or all of every feed will become dehydrated and stop gaining weight if it is not treated. The good news is that once it is identified, the treatment is well-established and the outcome is excellent.
Healthbooq covers newborn and infant health through the early months, including the small set of urgent conditions parents should know how to recognise.
What Is Actually Going On
The pylorus is the muscular valve at the bottom of the stomach. Partially digested food passes through it into the duodenum (the start of the small intestine). In pyloric stenosis, the muscle surrounding the pylorus progressively thickens and tightens over the first weeks of life, narrowing the passage until milk simply cannot get through. The stomach fills, contracts harder against the obstruction, and ultimately empties the only way available — backwards, with force.
The condition affects around 1 in 500 babies in the UK. It is 4–5 times more common in boys than in girls, more common in firstborn children, and runs in families — children of mothers who themselves had pyloric stenosis are at higher risk. The exact mechanism behind the muscle thickening is still not fully understood, though there is a clear genetic component.
A useful point: pyloric stenosis is not present at birth. The pyloric muscle is normal-sized in the first week or two; the thickening develops over the following weeks. That is why the typical presentation is not a baby who vomited from day one, but a baby who was feeding fine and then increasingly is not.
What It Looks Like
The vomiting in pyloric stenosis is projectile — not a dribble down the chin, not the milky possets that come up after most feeds, but a forceful ejection of a stomachful of milk that can travel a foot or more. The most distinctive feature is what happens immediately afterwards: the baby looks hungry and wants to feed again straight away. This cycle — full feed, complete vomiting, immediate hunger — is the pattern that experienced midwives and GPs recognise quickly.
Other clinical features:
- The vomit is not bile-stained. It looks like milk (sometimes curdled), not greenish-yellow. This is because the obstruction is upstream of where bile enters the gut. Green or yellow vomit in a young baby is a different and more urgent concern (possible malrotation with volvulus or other intestinal obstruction) — that needs immediate A&E assessment.
- The vomiting starts intermittently and becomes consistent. Over days to weeks the pattern shifts from occasional forceful vomits to one after every feed.
- Weight slows or drops. A baby who was tracking along their centile begins to flatten or fall.
- Dehydration develops. Fewer wet nappies, a sunken fontanelle, a very dry mouth, lethargy.
- Visible peristalsis. In some babies, you can see waves of muscular activity travelling across the abdomen after a feed — the stomach working against the obstruction.
- Palpable "olive." An experienced clinician can sometimes feel an olive-shaped mass in the right upper abdomen — the thickened pyloric muscle. This sign is now less relied upon since ultrasound has become routine, but historically it was the diagnostic clincher.
How It's Diagnosed
The investigation of choice is an abdominal ultrasound, which directly images the pylorus and measures the thickness and length of the muscle. A muscle wall over 4 mm thick and a pyloric channel over 16 mm long are the commonly used diagnostic thresholds.
Blood tests typically show a characteristic biochemistry: a hypochloraemic, hypokalaemic metabolic alkalosis. The repeated loss of hydrochloric acid from the stomach drives the chloride and potassium down and the bicarbonate up. These electrolyte abnormalities need to be corrected before surgery — operating on a dehydrated, alkalotic baby is more dangerous than the operation itself, which is why the surgical timing is calibrated around getting the biochemistry right rather than rushing to theatre.
How It's Treated
Pyloric stenosis is treated by pyloromyotomy — a procedure first described by Conrad Ramstedt in 1912 and substantially the same operation today. The surgeon makes an incision through the outer muscular layers of the pylorus down to the inner mucosal lining, allowing the channel to open without cutting into the gut itself. It can be done as a small open incision or laparoscopically; both have excellent outcomes.
The sequence:
- Diagnosis by ultrasound.
- Rehydration and correction of electrolytes with intravenous fluids — typically over 24–48 hours.
- Surgery, usually next on a planned list once biochemistry is corrected.
- Feeding restarted within a few hours of the operation, with small volumes that build up over the next 24 hours.
- Discharge typically within 24–48 hours of surgery.
Most babies feed normally within a few days, gain weight rapidly afterwards, and have no long-term consequences from the condition or the operation. Recurrence is rare. Some vomiting in the first day or two after surgery is common and not usually a sign of failed surgery.
When to Seek Help
A baby aged 2–8 weeks who is forcefully vomiting after feeds, who looks hungry again immediately, and who is not gaining weight (or losing weight) should be seen the same day. The route depends on how unwell the baby looks:
- GP same-day appointment if the picture is early and the baby still looks well.
- A&E if the baby is showing signs of dehydration (fewer than 4–6 wet nappies in 24 hours, sunken fontanelle, very dry mouth, lethargy, sunken eyes), or if vomiting is severe and constant.
- Emergency assessment immediately if vomit is green or yellow (bile-stained) at any point — this is a different problem and needs ruling out without delay.
Do not wait for the next health visitor appointment if the vomiting is worsening. Pyloric stenosis is one of the conditions where prompt assessment changes the trajectory significantly — and where, once recognised and treated, babies do extremely well.
Key Takeaways
Pyloric stenosis is a progressive thickening of the pylorus — the muscular valve at the stomach outlet — that gradually obstructs the passage of milk into the small intestine. It typically presents between 2 and 8 weeks of age with worsening forceful (projectile) vomiting after feeds, immediate hunger after vomiting, and eventually weight loss and dehydration. It is around 4–5 times more common in boys, more common in firstborn babies, and has a genetic component. Diagnosis is by abdominal ultrasound. Treatment is surgical (pyloromyotomy), curative, and outcomes are excellent — but the baby needs to be rehydrated and electrolytes corrected first, which is why prompt recognition matters.
