Pyloric Stenosis in Babies: Signs, Diagnosis, and Treatment

Pyloric stenosis is one of the most common surgical conditions of early infancy, affecting approximately 1 in 500 babies in the UK. Despite being relatively common, many parents have never heard of it — and because its main symptom (vomiting) is so ubiquitous in babies, it is often not recognised immediately. Understanding the distinctive pattern of pyloric stenosis can mean the difference between an early diagnosis and weeks of unnecessary weight loss and distress.

What Is Pyloric Stenosis?

The pylorus is the muscular valve at the base of the stomach that controls the passage of partially digested food (chyme) into the small intestine. In pyloric stenosis, this muscle becomes abnormally thickened and narrowed (stenosis means narrowing), preventing milk from passing through into the intestine.

The result is that milk builds up in the stomach until there is no more room, at which point it is expelled — dramatically — backwards, out of the mouth.

Pyloric stenosis is not present at birth in most affected babies. The muscle thickening develops over the first few weeks of life, which is why symptoms typically begin between 3 and 6 weeks of age, occasionally a little earlier or later.

Who Is Affected?

Pyloric stenosis is:

• More common in boys than girls, with a male-to-female ratio of approximately 4:1. The reason for this sex difference is not fully understood.
• More likely in firstborn children, particularly firstborn males.
• More common in bottle-fed babies than breastfed babies, though the reason for this association is not entirely clear.
• Associated with a family history: if a parent had pyloric stenosis, the risk in their child is higher than in the general population.
• More common in babies born to mothers who smoked during pregnancy.

Pyloric stenosis can affect babies of any ethnicity, though there are variations in incidence across different populations.

Recognising the Symptoms

Projectile Vomiting

The defining feature of pyloric stenosis is projectile vomiting — vomiting of unusual force that travels a significant distance, sometimes across the room. This distinguishes it from the common possetting or reflux that affects many babies, which tends to be gentle dribbling or mild regurgitation.

In pyloric stenosis, the vomiting:

• Is forceful enough to be genuinely striking — many parents describe their first episode as alarming
• Typically occurs within 30 minutes of a feed
• Is non-bilious (not green or yellow) — the obstruction is above the point where bile enters the intestine, so the vomit contains only milk or partially digested milk
• Is progressively worsening — in the early stages it may be less dramatic, but it becomes more forceful as the pyloric muscle thickens further

Hunger After Vomiting

A characteristic feature that helps differentiate pyloric stenosis from viral illness is that babies with pyloric stenosis typically remain hungry after vomiting. They have, in effect, just emptied their stomach and are keen to feed again. Parents often describe their baby as ravenous, feeding eagerly and then vomiting again shortly afterwards.

Weight Loss and Poor Growth

Because food is not reaching the intestine, babies with pyloric stenosis fail to absorb nutrients and begin to lose weight. A baby who was gaining well and then begins to lose weight, in combination with forceful vomiting, should be assessed urgently.

Signs of Dehydration

As pyloric stenosis progresses without treatment, dehydration and electrolyte imbalance become significant risks. Signs of dehydration in a baby include:

• Fewer wet nappies than usual
• Dark, concentrated urine
• Dry mouth and eyes
• Sunken fontanelle (the soft spot on top of the head)
• Lethargy and reduced alertness

Dehydration in young babies can develop rapidly and requires urgent medical attention.

Visible Stomach Contractions

In some affected babies, wave-like contractions of the stomach muscle can be visible through the abdominal wall, particularly after a feed. This is called visible peristalsis and, when present, is a strong indicator of pyloric stenosis.

When to Seek Medical Attention

If your baby is vomiting forcefully after every feed and not regaining birth weight or losing weight, see your GP that day or attend an emergency department. Pyloric stenosis is a surgical emergency and becomes more serious the longer it goes untreated.

If your baby shows any signs of dehydration or appears unusually lethargic or unwell, do not wait for a GP appointment — go directly to A&E.

Diagnosis

Pyloric stenosis is diagnosed using ultrasound of the abdomen, which allows direct visualisation of the thickened pyloric muscle. An experienced sonographer can measure the length and thickness of the pylorus; values above established thresholds (typically pyloric length greater than 15–16mm and muscle wall thickness greater than 3–4mm) are diagnostic.

Ultrasound has largely replaced the older "test feed" and palpation approach that was previously used to feel for the "pyloric olive" (the palpable thickened muscle), though an experienced clinician may still feel for this.

Blood tests will be performed to assess electrolytes. Pyloric stenosis causes a specific pattern of electrolyte disturbance — hypochloraemic, hypokalaemic metabolic alkalosis — due to the loss of hydrochloric acid through vomiting. This needs to be corrected before surgery is safe.

Treatment: Pyloromyotomy

The treatment for pyloric stenosis is surgical. The operation — called a Ramstedt pyloromyotomy — involves cutting through the outer muscle layer of the pylorus to allow it to open normally. The inner lining of the pylorus (the mucosa) is left intact.

The surgery is typically performed laparoscopically (via small keyhole incisions), though open surgery is also performed at some centres. It is a highly effective, well-established procedure with an excellent safety record.

Before surgery: Any dehydration and electrolyte imbalances must be corrected with intravenous fluids. This correction can take several hours and is not a stage to rush, as operating on a severely dehydrated or metabolically disturbed baby carries additional risk.

The operation itself: Laparoscopic pyloromyotomy typically takes 20–30 minutes under general anaesthetic.

Recovery

Recovery from pyloromyotomy is generally swift. Most babies are able to feed within a few hours of surgery, beginning with small amounts of water or dilute milk and gradually building back to normal feeding volumes over 24–48 hours. A small amount of vomiting is normal in the first day or two after surgery as the stomach and pylorus recover.

Most babies are discharged home within 1–2 days of surgery. Parents are usually advised that the baby can return to normal feeding within a week, and the prognosis after successful surgery is excellent — there are no long-term complications in the vast majority of cases.

The surgical scar is small and fades well. If the operation is performed laparoscopically, the scars are typically around the umbilicus and are often barely visible by the time the child reaches adulthood.

Long-Term Outlook

The long-term outlook following pyloromyotomy is excellent. Pyloric stenosis does not recur after successful surgery, and the vast majority of affected babies grow and develop entirely normally.