Baby Vomiting and Diarrhoea: Gastroenteritis, Dehydration, and When to Worry

Gastroenteritis in babies is alarming but usually short-lived. The key concern is dehydration — babies have a smaller fluid reserve than older children and can become dehydrated more quickly. Here's how to manage it.

Is it gastroenteritis or something else?

Normal infant vomiting (posseting): Bringing up a small amount of milk after feeds is common and normal, especially in the first few months. It shouldn't cause distress and baby should be gaining weight normally.

Reflux: Frequent, effortless regurgitation. Often related to immature oesophageal sphincter. See our reflux guide.

Gastroenteritis: Vomiting usually starts suddenly, often with diarrhoea, sometimes fever. Baby looks unwell.

Signs that need urgent assessment (not gastroenteritis):

• Forceful projectile vomiting in a baby under 8 weeks — particularly if it occurs after every feed and baby is hungry again immediately. This can indicate pyloric stenosis, where the muscle at the stomach exit is thickened, blocking emptying. Requires surgical treatment.
• Vomiting with a distended (swollen) abdomen and no poo
• Blood in vomit
• Bile (green/yellow) in vomit in a newborn — always urgent

Gastroenteritis: what to expect

Vomiting typically lasts 1–3 days, improving each day. Diarrhoea often follows vomiting and can persist for up to 7–10 days (and sometimes longer after rotavirus).

The most common causes in babies:

• Rotavirus — the UK rotavirus vaccine (given at 8 and 12 weeks) significantly reduces this
• Norovirus — highly contagious, affects all ages
• Other viral gastroenteritis
• Less commonly, bacterial causes (Salmonella, Campylobacter)

The priority: preventing dehydration

Dehydration is the main risk. Signs of dehydration in babies:

Mild dehydration:

• Slightly dry lips and mouth
• Fewer wet nappies than usual (less than 4 in 24 hours)
• Less active than usual

Moderate dehydration — seek medical attention:

• Dry mouth and lips
• No tears when crying
• Very few wet nappies (1–2 in 24 hours)
• Sunken fontanelle (the soft spot on baby's head appears more dipped than usual)
• Sunken eyes
• Pale, mottled skin

Severe dehydration — call 999:

• No wet nappies in 8+ hours
• Sunken eyes and very dry mouth
• Cold extremities
• Abnormal drowsiness, limpness, unresponsiveness

How to manage fluids

Breastfed babies: Continue breastfeeding through illness. Offer the breast more frequently than usual. Breast milk provides hydration, nutrition, and immune support — it's the best fluid for a breastfed sick baby.

Formula-fed babies: Continue formula feeds as normal unless baby is vomiting significantly. If vomiting frequently:

• Offer oral rehydration solution (ORS) such as Dioralyte between feeds
• Do NOT dilute formula — a common instinct, but dangerous as it doesn't provide adequate electrolytes

Solid foods: If baby has been eating solids, continue to offer food as tolerated. Plain, easily digestible foods (toast, rice, banana, mashed potato). No need for a special "BRAT diet" — offer a range of foods.

What NOT to give:

• Fruit juice — high sugar worsens diarrhoea
• Sports drinks (Lucozade, Gatorade) — not appropriate electrolyte balance for babies
• Flat lemonade — an old remedy with no evidence and excess sugar

Oral rehydration solution (ORS)

Dioralyte (sachets mixed with water) or Hydralyte are the recommended rehydration solutions for babies. They contain the correct balance of electrolytes to replace what's lost.

Use if:

• Baby is showing mild dehydration signs
• Vomiting after formula feeds and you're concerned about fluid loss
• Diarrhoea is significant and prolonged

How to offer: small, frequent amounts — a teaspoon every minute or two if baby is vomiting. Syringe or teaspoon rather than a bottle often works better.

Hygiene to prevent spread

Gastroenteritis spreads easily. Key steps:

• Hand washing with soap and water after every nappy change and before preparing food — alcohol gel is NOT effective against norovirus and rotavirus
• Wash soiled clothing and bedding on a hot cycle
• Keep sick adults and children away from food preparation
• If baby has norovirus, no childcare/nursery for 48 hours after the last vomiting or diarrhoea episode

When to call your GP or 111

• Any age: Signs of moderate or severe dehydration (see above)
• Under 3 months: Any vomiting illness needs same-day assessment
• Vomiting lasts more than 24 hours in babies under 6 months, or 48 hours in older babies
• Diarrhoea lasts more than 7 days
• Blood in vomit or stool
• High fever (39°C+) that doesn't respond to paracetamol
• Baby is unusually lethargic, pale, or unresponsive
• You're worried

For projectile vomiting in a baby under 8 weeks: Contact your GP or go to A&E urgently.