Dehydration in babies: how to spot it and what to do
When your baby is unwell and not feeding well, it is easy to feel helpless. Dehydration is one of the things parents worry about most during illness, and rightly so, because babies can lose fluid quickly and may not show obvious signs until things have progressed. The good news is that the signs are recognisable once you know what to look for, and most cases are straightforward to manage at home if you act early. This guide helps you understand the signs, know when to call for help, and feel confident about what to do.
Why babies dehydrate so quickly
Babies are not simply small adults when it comes to fluid balance. A much higher proportion of a baby's body weight is water compared with an older child or adult, and their surface-area-to-weight ratio is larger, which means they lose moisture through their skin and breathing at a faster rate relative to their size.
They also lose fluid through urine and stool continuously, and unlike you, they cannot regulate this well or ask for a drink when they feel thirsty. Any illness that brings vomiting, diarrhoea or fever dramatically speeds up that fluid loss. A baby who is refusing feeds during illness can tip into dehydration within hours, not days. This is why watching for early signs matters so much.
What causes dehydration in babies
The most common cause is a stomach bug (gastroenteritis) that causes both vomiting and diarrhoea. Either one alone can cause fluid loss, but together they are particularly fast-acting in a small baby.
Other causes include:
- Fever: a raised temperature increases fluid loss through sweating and faster breathing, even if your baby is not vomiting.
- Refusing feeds: a baby who is in pain, congested, or simply too tired and unwell may feed much less than usual, which cuts off their main source of fluid intake.
- Hot weather: sweating in a warm environment without extra feeds means more fluid goes out than comes in.
- A combination: a stomach bug in summer heat, or a feverish baby who is also refusing feeds, multiplies the risk considerably.
Early signs to watch for
The earlier you spot dehydration, the easier it is to manage. These are the first signs to look out for:
- Fewer wet nappies than usual. A healthy baby under six months should have at least six wet nappies in 24 hours. Fewer than four is a warning sign.
- Dark yellow urine. In older babies who have started solids and water, darker urine than usual is a sign that the body is conserving fluid.
- Dry or sticky mouth and lips. A well-hydrated baby's mouth should feel moist; stickiness or dryness is an early clue.
- No tears when crying. If your baby is clearly distressed but there are no tears, take note.
- Unsettled or slightly drowsy. A baby who seems more irritable or less alert than their normal self during illness may be showing an early effect of dehydration.
None of these signs alone means a crisis, but two or more together during illness means you should act: offer feeds more frequently and watch closely over the next few hours.
Late signs that need urgent attention
If early dehydration is not addressed, it can progress quickly to a more serious level. These are the signs that mean you need to call for help immediately:
- The fontanelle (the soft spot on the top of your baby's head) is visibly sunken inward rather than sitting flat.
- Eyes look sunken or have dark rings around them.
- Skin looks pale, mottled or blotchy.
- Skin stays tented when you gently pinch it, rather than springing back immediately.
- Hands and feet feel cold even if your baby has a fever elsewhere on their body.
- Your baby is very drowsy, floppy or difficult to wake.
- Your baby has not had a wet nappy for six or more hours.
If you see any of these signs, call 999 or go to A&E immediately. Do not wait to see whether things improve on their own.
How to count wet nappies
Nappy counting is the single most practical way to monitor hydration at home. Here is what to expect at different stages:
- Newborn period (days one to five or six): roughly one wet nappy per day of life until your milk supply is fully established. Day one: one nappy. Day two: two nappies, and so on.
- Once feeding is established: at least six wet nappies in 24 hours for both breastfed and formula-fed babies.
One practical note: modern nappies are highly absorbent and can feel dry even when they have been used. If you are not sure, place a small piece of tissue inside the nappy to check. A significant drop from your baby's normal pattern, rather than an absolute number, is often the clearest signal.
Tracking feeds and nappies in an app like Cubby makes it much easier to notice a sudden drop. When you can see that your baby had eight wet nappies yesterday and only two today, that is far more useful than trying to remember.
Keeping your baby hydrated during illness
What you give your baby during illness depends on their age and how they are fed.
- Breastfed babies: offer the breast more frequently than usual. Breast milk contains the right balance of fluid and electrolytes alongside antibodies that help fight the illness. Do not stop breastfeeding during a stomach bug.
- Formula-fed babies: continue with normal-strength formula. Never dilute formula, as this alters the electrolyte balance. Offer smaller amounts more frequently if your baby is struggling to keep feeds down.
- Babies over six months: small sips of cooled boiled water between feeds are fine to top up fluids, but milk feeds should remain the main source.
What not to give:
- Plain water for babies under six months: this can cause hyponatraemia, a dangerous drop in blood sodium. Breast milk or formula is the right choice at this age.
- Fruit juice or sports drinks at any age during illness: these have the wrong balance of sugars and salts and can make diarrhoea worse.
- Oral rehydration salts (ORS) such as Dioralyte without guidance: ORS can be very helpful but should only be used when a doctor or pharmacist has specifically recommended them for your baby and given you instructions on how to use them.
When to call for help
Call 999 or go to A&E immediately if your baby has any of the late signs of dehydration listed above, or if you are seriously worried at any point and feel your baby is deteriorating.
Call 111 or your GP if your baby has had fewer than four wet nappies in 24 hours, has vomited more than three times in a row, has diarrhoea lasting more than 24 hours (or any diarrhoea in a very young baby), or if you are unsure what to do and want guidance.
A baby under three months with any sign of dehydration should be seen urgently. Do not wait for the situation to worsen before making that call.
What happens in hospital
If your baby is assessed at hospital, the team will check several things to gauge how dehydrated they are: the fontanelle, skin elasticity, the eyes, the moisture of the mouth and lips, heart rate and other vital signs.
For mild to moderate dehydration, the usual treatment is oral rehydration solution given in small, frequent amounts. If your baby cannot take this by mouth, it can be given through a thin tube passed gently down the nose into the stomach (a nasogastric tube).
For severe dehydration, intravenous fluids are given through a drip. This sounds alarming but it works quickly and effectively. Most babies with dehydration recover fully and quite fast once fluids are replaced, and many are home the same day.
Frequently asked questions
How many wet nappies should my baby have?
A breastfed or formula-fed baby should have at least six wet nappies in 24 hours once your milk supply is established (usually from day five or six). In the newborn period, expect roughly one wet nappy per day of life until then. Fewer than four wet nappies in 24 hours is a warning sign worth acting on.
Is it safe to give water to a baby under six months?
No. Babies under six months should not be given plain water. Their kidneys cannot process it properly, and giving water can dilute the sodium in their blood to dangerous levels, a condition called hyponatraemia. Breast milk or normal-strength formula provides all the fluid they need.
What are oral rehydration salts and when do I need them?
Oral rehydration salts (ORS) such as Dioralyte are a pre-mixed balance of salts and glucose designed to replace what is lost through vomiting or diarrhoea. They are not a first step to reach for at home without guidance. Use them only when a doctor or pharmacist has recommended them for your baby, and always follow the preparation instructions exactly.
How do I check if my baby's fontanelle is sunken?
The fontanelle is the soft spot on the top of your baby's head. When your baby is calm and sitting upright, it should sit flat or very slightly raised. If it dips visibly inward, that is a sunken fontanelle and is a late sign of dehydration. Note that the fontanelle naturally bulges a little when a baby cries, so check it when your baby is settled.
How quickly can babies get dangerously dehydrated?
Babies can move from mild to significant dehydration within hours during a bout of vomiting or diarrhoea, particularly if they are also refusing feeds. Their small body size means even a modest loss of fluid is proportionally large. This is why early signs such as fewer wet nappies matter and why you should not wait until your baby looks very unwell before acting.
Can I make oral rehydration solution at home?
Home-made sugar and salt solutions are not recommended for babies. Getting the ratio wrong can cause more harm than good. If a doctor recommends ORS, use a pharmacy product such as Dioralyte which is accurately formulated. If you cannot get to a pharmacy quickly, call 111 for advice rather than mixing something at home.
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