Newborn Diarrhoea Explained: Causes and When to Worry

It can be worrying when your newborn’s nappies don’t look quite as you expect. New parents often find themselves asking: “Is this normal, or should I call the doctor?” One of the most common concerns is diarrhoea. Since babies’ stools can vary a lot in colour, texture, and frequency, it can be hard to know what’s typical and what signals a problem. This guide explains newborn diarrhoea – causes, when it’s nothing to worry about, and when it’s a sign you should seek medical help.

What Does Normal Baby Poo Look Like?

Newborn poo changes quickly in the first weeks of life.

• Meconium: Thick, sticky, dark green or black stools passed in the first few days.
• Breastfed babies: Yellow, mustard-like stools that are soft or runny and can appear seedy.
• Formula-fed babies: Firmer, tan or brown stools that may smell stronger.

Normal baby stool can be loose without being diarrhoea. The key is to notice sudden changes or patterns that don’t fit your baby’s usual routine.

What Counts as Newborn Diarrhoea?

Diarrhoea in newborns usually means:

• Frequent, watery stools that are looser than usual.
• An increase in the number of nappies filled per day.
• Stool that leaks through nappies or soaks clothing.

Unlike normal loose stools in breastfed babies, diarrhoea tends to be watery, explosive and more frequent.

ALSO READ: It’s Time To Talk Poop: How To Help Your Child With Constipation

Common Causes of Newborn Diarrhoea

Infections

Viral or bacterial infections can cause diarrhoea. Symptoms may include fever, irritability or vomiting alongside watery stools.

Feeding changes

Switching from breast to formula, changing formula brands, or introducing new foods (for older infants) can upset your baby’s tummy.

Allergies and intolerances

Some babies react to cow’s milk protein, lactose, or other ingredients in formula. Breastfed babies may react to something in mum’s diet.

Antibiotics

If your baby is prescribed antibiotics, it may disrupt the balance of gut bacteria, causing looser stools.

Teething (older babies)

Teething doesn’t directly cause diarrhoea, but increased saliva swallowed can sometimes make stools looser.

When to Worry About Newborn Diarrhoea

Call your doctor or health visitor if you notice:

• Signs of dehydration: dry mouth, sunken eyes, fewer wet nappies, drowsiness.
• Blood or mucus in stools.
• High fever or repeated vomiting.
• Diarrhoea lasting more than a day or two.
• A sudden change in stool colour (for example, very pale or red).

Newborns can become dehydrated quickly, so it’s always best to get medical advice early if you’re concerned.

How to Care for a Newborn with Diarrhoea

• Keep them hydrated: Continue breastfeeding or formula feeding as normal. Offer feeds more often if possible.
• Monitor nappies: Count wet nappies to track hydration.
• Watch for rashes: Frequent diarrhoea can cause nappy rash, so change nappies quickly and use barrier cream.
• Seek advice early: Even if symptoms seem mild, checking with your GP or midwife is sensible for young babies.

Preventing Diarrhoea in Newborns

• Practise good hygiene: wash hands before and after nappy changes and before feeding.
• Ensure bottles and teats are sterilised properly.
• If breastfeeding, be aware of foods in your own diet that may upset your baby.
• Avoid unnecessary changes in formula unless recommended by a healthcare professional.