Dehydration in babies: signs, causes and how to help

Dehydration simply means that your baby is losing more fluid than they are taking in. As well as water, the body loses important salts known as electrolytes, which it needs to work properly.

Babies get most of their fluid through their feeds. For young babies, that means breastmilk or correctly prepared first infant formula. Once solid foods are introduced at around six months, babies can also begin having small amounts of water, although milk remains an important source of both nutrition and hydration throughout the first year.

Problems can arise when that balance changes. A baby with vomiting or diarrhoea, for example, may be losing more fluid than usual at exactly the same time as they are feeling too unwell to feed normally.

Babies are also still developing their immune defences, so illnesses involving fever, vomiting or diarrhoea are particularly important to keep an eye on.

One of the difficulties with dehydration in babies is that they can’t simply tell us they’re thirsty. Instead, we have to look for changes in the things we can see, particularly their feeding, wet nappies, behaviour and general wellbeing.

Their small body size and relatively high fluid needs also mean that changes can become important more quickly than they would in an older child. This is where knowing your own baby is incredibly useful.

Parents sometimes get very focused on counting nappies, but I’m also interested in the change. If your baby normally produces really heavy wet nappies and suddenly you’re picking up nappies that feel almost dry, that tells us something, even if you can still say they’ve technically had several wet nappies that day.

In the newborn period, we expect the number of wet nappies to increase over the first few days. From around day five, a well fed baby would usually be producing at least six heavy wet nappies in 24 hours. If this isn’t happening, particularly alongside concerns about feeding, speak to your midwife or health visitor.

There isn’t one single symptom that tells us a baby is definitely dehydrated. Usually, we build a picture from several signs and, importantly, how different things are from that baby’s normal pattern.

You may notice:

There are times when you shouldn’t simply continue monitoring at home.

Seek urgent medical advice if:

If your baby is very difficult to wake, has difficulty breathing, develops blue, grey, very pale or blotchy skin, or seems seriously unwell, seek emergency medical help.

And remember, you don't have to prove your baby is dehydrated before asking someone to assess them. If you are worried about a young baby, it is absolutely appropriate to seek advice.

Dehydration usually happens because a baby is either losing more fluid than usual, taking less fluid than usual, or both.

Vomiting and diarrhoea are common causes because fluid is being lost directly from the body. Fever and hot weather can increase fluid requirements, while illnesses can make babies less interested in feeding.

Sometimes the reason is less obvious. A baby with a blocked nose may find feeding harder because coordinating sucking and breathing is uncomfortable. Another baby may have a sore mouth or throat and suddenly start refusing feeds.

Feeding issues and dehydration

For young babies, feeding and hydration are very closely connected because virtually all their fluid comes from their milk.

With breastfeeding, a baby can spend a long time at the breast without necessarily transferring milk effectively. This is why, particularly with newborns, we don’t assess feeding simply by asking how many minutes a baby breastfeeds. We look at positioning and attachment, swallowing, weight, nappies and how the baby behaves during and after feeds.

The same principle applies to bottle feeding. If a baby suddenly takes much less than normal or repeatedly vomits their feeds, keep an eye on their nappies and general wellbeing rather than looking at one bottle in isolation.

Formula should always be prepared exactly according to the manufacturer's instructions. Adding too much powder makes a feed too concentrated and can contribute to dehydration. Adding extra water isn't a safe solution either because this changes the nutritional and electrolyte balance of the feed.

If your baby is repeatedly vomiting, having diarrhoea or struggling with feeds and you are wondering whether their formula agrees with them, speak to your health visitor or GP rather than repeatedly changing formulas yourself.

Illness related causes

Stomach bugs are probably the most obvious illness associated with dehydration because babies can lose fluid through both vomiting and diarrhoea.

But other illnesses can affect hydration too. A baby with a fever may need more fluid while simultaneously feeling less interested in feeding. A cold can make feeding harder because of nasal congestion. Sometimes a baby manages only short feeds before becoming tired and stopping.

In practice, an unwell baby doesn't always feed in the way you're used to. They may latch for a few minutes and come off or take a small amount from their bottle and then want some more half an hour later.

That's why I wouldn't judge hydration from one feed. I'm interested in what they're managing over time, alongside their nappies and how well they seem.

If you think your baby is significantly dehydrated, particularly if they are very young or unwell, seek medical advice rather than trying to treat them yourself at home.

If the changes are mild and your baby is otherwise alert and able to feed, continue offering their normal milk frequently while keeping a close eye on their nappies and behaviour.

An unwell baby may not want their usual full feed, and that's where smaller, more frequent feeds can sometimes be easier for them to manage. If they repeatedly vomit feeds, cannot keep fluids down or their wet nappies are reducing, seek professional advice.

When to seek medical advice

You never need to be certain that your baby is dehydrated before contacting a healthcare professional.

I'd want parents to seek advice if their baby has significantly fewer wet nappies, is taking much less milk than usual, repeatedly vomits, has ongoing diarrhoea, becomes unusually sleepy or irritable, or has a dry mouth, sunken eyes or a sunken fontanelle.

I'd also seek advice if things aren't improving or if several smaller changes are starting to add up. And there is one thing that is difficult to put on a symptom checklist: you know your baby.

I've spoken to many parents who haven't been able to identify one dramatic symptom but have known their baby wasn't behaving normally. That matters. If your instinct is telling you something isn't right, you don't need to wait until your baby ticks enough boxes on a list before asking for help.

Hot weather is another time when parents naturally start worrying about how much their baby is drinking.

An exclusively breastfed baby under six months doesn't usually need additional water. You may notice they want to breastfeed more frequently, and that's absolutely fine.

Formula-fed babies under six months should continue their normal milk feeds. In very hot weather, they may sometimes need small amounts of cooled boiled water in addition to their usual formula.

Once your baby is around six months and has started solids, you can offer water regularly as well as continuing their milk feeds.

Hydration isn't just about drinks, though. Keeping your baby comfortable and cool helps too. Dress them appropriately for the temperature, keep them out of direct sunlight and be particularly mindful of how hot enclosed spaces such as cars and prams can become.

Keep an eye on nappies during hot weather too. Again, I would be looking for a noticeable change from what's normal for your baby rather than obsessively counting every nappy.

Dehydration in babies can develop more quickly than it does in older children, which is why recognising changes early matters.

Wet nappies are one of the most useful things you can monitor, but don't look at nappies in isolation. Feeding, behaviour, alertness and how your baby seems generally all help build the picture.

Continue offering appropriate milk feeds, pay attention to changes from your baby's normal pattern and ask for help if symptoms persist or worsen.

You don't need to wait until you're certain your baby is dehydrated. If something doesn't feel right, seeking advice is always reasonable.