Diarrhoea in Babies: Is Your Baby's Poo Too Runny?

Opening a nappy to find a sudden, watery explosion can instantly shift parental gears from calm to concerned. Is it just a very loose poo, or is this actually baby diarrhoea? It’s a question that can send a shiver down any parent’s spine because knowing what to do when baby has diarrhoea and how to prevent dehydration in babies is critical. You might find yourself instinctively reaching for your phone, searching frantically for answers.

The good news is you don’t have to tackle this worry alone. With the Aptaclub AI Poo Checker, you can get instant analysis of what’s in your baby’s nappy from a simple photo upload. Our tool helps you understand changes in their nappy contents and provides reassurance when you need it most. So, if you’ve got questions about your baby’s poo, you can just Poogle It! If your baby has repeated episodes of explosive diarrhoea, especially if it’s a change from their usual poo pattern, it’s best to seek prompt medical advice.

Let’s explore the signs of infant diarrhoea, common causes, and crucial steps you can take to keep your little one safe and comfortable.

First things first: what is baby diarrhoea? For babies, especially breastfed babies, runny poo can be quite normal. The key isn't just how watery it is, but also changes in frequency, volume, and consistency from your baby's usual pattern1.

Given that baby poo comes in a variety of colours and consistencies, telling the difference between a normal stool and diarrhoea can be a challenge. It all comes down to understanding your baby’s unique bowel habits.

What's 'normal' varies significantly. If your baby is breastfed, they might have a soft, loose poo after every feed or go as little as once a week. Formula-fed babies usually have slightly firmer stools and may poo a few times a day or as little as once daily as they get older¹¹. This wide range is why a sudden change is the most reliable indicator of a problem. 

Diarrhoea is characterised by frequent, very loose, or watery poos that are a distinct change from your baby's typical bowel movements. These stools can be explosive, often have a foul smell, and may contain mucus or even streaks of blood [3, 124]. Whether your baby is breastfed or formula-fed, if they are pooing much more than they usually would, it could be a sign of tummy trouble. Trust your instincts about what’s normal for your baby.

Infant diarrhoea is often a sign that something is irritating your baby's digestive system. Here are the most common culprits:

The most frequent cause of baby diarrhoea is a viral infection, like rotavirus (though many babies are now vaccinated against this)⁸ or other common viruses. Bacterial infections can also cause it, though less commonly. These infections irritate the lining of the gut, leading to increased fluid secretion and rapid transit of nappy contents⁴.

Sometimes, baby diarrhoea can be a symptom of a food sensitivity or allergy, such as a cow's milk protein allergy⁵. This might be more noticeable when introducing new foods during weaning or when switching from breastmilk to infant formula. If the diarrhoea is persistent and accompanied by other symptoms like a rash, excessive crying, or fussiness, a dietary link should be considered.

Certain medications, especially antibiotics, can disrupt the natural balance of bacteria in your baby’s gut, leading to diarrhoea as a side effect. Occasionally, diarrhoea can be caused by underlying medical conditions, but infections and dietary factors are far more common.

If you’re worried about your baby’s diarrhoea, always speak to your GP or health visitor.

While watery poo is the most obvious sign, accompanying symptoms are crucial for assessing the severity of baby diarrhoea.

The biggest risk with baby diarrhoea is dehydration³. Babies can become dehydrated very quickly, so it’s essential to recognise the signs:

• Fewer wet nappies than usual.
• Dry mouth and tongue.
• No tears when crying.
• Sunken fontanelle (the soft spot on a baby’s head).
• Sunken eyes.
• Lethargy or unusual sleepiness.
• Cool, blotchy hands or feet.

If you notice any signs of infant dehydration, seek immediate medical attention from your doctor or call 111 (NHS non-emergency number in the UK).

Watch out for:

• Fever: A high temperature alongside diarrhoea suggests an infection.
• Vomiting: Frequent vomiting with diarrhoea significantly increases the risk of dehydration.
• Mucus or Blood in Stool: While occasionally seen with severe straining or mild irritation, significant amounts of mucus or visible blood in the stool always warrant medical review⁶.
• Irritability or Fussiness: Your baby might be uncomfortable due to stomach cramps.

There’s no one size fits all when it comes to how long your baby’s diarrhoea will last. In general terms, diarrhoea will last anywhere from a couple of days up to a week¹⁰. If your baby’s symptoms haven’t cleared by then, always consult your GP for further advice. 

Most babies’ diarrhoea improves within a few days and usually clears within a week. If it doesn’t get better, or if your baby also has symptoms like fever, vomiting, blood in their stool, or signs of dehydration — seek medical advice right away.

Whilst it’s very common to associate teething with diarrhoea, the NHS and HSE is clear that there’s no evidence to support this⁹.

Acting quickly can make a big difference in preventing infant dehydration and ensuring your baby’s comfort.

The most critical step is to replace lost fluids. Keep offering feeds frequently.

• Breastfed Babies: Continue breastfeeding on demand, offering more frequent, shorter feeds. Breastmilk is incredibly protective and hydrating⁷.
• Formula-Fed Babies: Continue to offer their usual infant formula. If they are vomiting frequently or showing signs of dehydration, your GP might recommend oral rehydration solutions (ORS). Follow their advice on how to use these carefully.
• Older Babies (6 months+): In addition to milk, you can offer small sips of cooled, boiled water or age-appropriate ORS³. Avoid fruit juices or sugary drinks, as these can sometimes worsen diarrhoea and are not a suitable drink for infants under 12 months of age.

Do not stop feeding your baby. Continue to offer your baby their usual meals, based on appetite. If they prefer to eat smaller amounts, soft and easy-to-eat foods like bananas, bread, rice, or lentils can be helpful for babies that are weaning. You can include protein and vegetables if they’re willing. There’s no need to restrict their diet.  A return to variety and normal meals is encouraged as soon as they’re feeling better.

While we want to empower you with knowledge, some situations with baby diarrhoea are urgent.

Always seek immediate medical advice if:

• Your newborn poop is watery (under 3 months old) and has diarrhoea.
• You notice any signs of dehydration in your baby.
• There is blood or a lot of mucus in their poo.
• Your baby has a high fever (especially if very young).
• Your baby is very drowsy, unresponsive, or unusually irritable.
• Diarrhoea is persistent and doesn't improve within 24-48 hours or seems to be getting worse³.

Trust your instincts. If you are worried, it is always best to speak to your GP or call 111.

Moments of concern like baby diarrhoea highlight the importance of clear, reliable information. Aptaclub's AI Poo Checker is designed precisely for these moments, aiming to reduce parental anxiety and provide science-backed answers to what’s in your baby's nappy.

Our unique AI-powered web tool* offers instant analysis of your baby's poo by simply uploading a photo from your phone⁸. Built on a huge database of 3.5 million nappies and developed by our experts following 50 years of research in infant nutrition and immune systems, it helps you identify poo colours and stool consistency, effortlessly track changes over time, and gain parental reassurance with answers within seconds^{7, 9}. It empowers you with detailed guidance and handy stool charts, helping you understand the meaning of your baby’s stool and supporting confident discussions with your healthcare professional.

Whatever you find in your baby’s nappy, our tool can give you a quick, reliable analysis in a snap, and help you track changes to show your doctor.

^{*The Aptaclub AI Poo Checker is not a diagnostic tool and does not provide medical insights about your baby’s digestive health. It does not replace the advice of a healthcare professional.}