Dehydration in babies: signs to watch and when to act

Babies have a higher proportion of body water than older children and adults, and they lose fluids faster relative to their body weight — through breathing, sweating, and any illness involving vomiting or diarrhoea. This makes dehydration a real risk when babies are unwell, and one that can deteriorate faster than parents expect.

Knowing the signs at each severity level — and knowing exactly when to call for help — is the most useful thing you can do.

Why dehydration happens

Common causes in infants:

• Vomiting or diarrhoea — the most common cause; both lead to rapid fluid loss
• Fever — increases fluid loss through sweating and faster breathing
• Refusing feeds — whether due to illness, mouth pain (teething, thrush, HFMD sores), or other causes
• Hot weather / overheating — increased fluid loss through sweat
• Breastfeeding difficulty — supply issues or poor latch can mean inadequate intake even when the baby appears to be feeding

Signs of dehydration by severity

Mild dehydration

• Fewer wet nappies than normal — a baby who normally produces 6+ wet nappies per day dropping to 4 or fewer is a warning sign¹
• Slightly dry mouth and lips
• Less active than usual — quieter, slightly less alert
• Slightly darker urine (if you can observe it — pale yellow is normal; darker yellow indicates concentration)

At this stage, the right response is to increase feeding (breast or formula as normal) and monitor closely.

Moderate dehydration

• 3 or fewer wet nappies in 24 hours
• Sunken fontanelle — the soft spot on top of the head dips inward rather than lying flat or slightly raised
• No tears when crying — a baby who usually cries with tears is not producing them
• Dry mouth and sticky mucous membranes
• Sunken eyes
• Increased sleepiness or lethargy beyond what the illness explains

At this stage, a baby under 6 months should be assessed by a doctor — do not wait to see if it resolves. Call your GP or 111 (UK) / pediatrician or urgent care (US).¹

For babies over 6 months with moderate dehydration, your GP may advise oral rehydration solution (ORS such as Dioralyte in the UK or Pedialyte in the US) in small, frequent amounts. Do not make your own rehydration solution at home; the electrolyte balance is critical and homemade versions can be dangerous.²

Severe dehydration

Severe dehydration is a medical emergency.

• No wet nappy for 8 hours or more
• No tears, very dry mouth
• Deeply sunken eyes and fontanelle
• Cold, mottled, or pale extremities — hands and feet feel cold and look blotchy or pale even if the baby has a fever
• Very lethargic, floppy, or difficult to wake
• Rapid or laboured breathing
• No urine output

Call 999 (UK) / 911 (US) immediately or go directly to the nearest emergency department.¹

Do not attempt to manage severe dehydration at home. Intravenous fluids and medical assessment are needed.

Under 6 months: a lower threshold

For babies under 6 months, any dehydration concern should prompt immediate contact with your GP or health visitor — not a wait-and-see approach.¹ Younger babies dehydrate faster, their signs can be subtler, and they cannot be given ORS without medical direction.

Specifically:

• Under 3 months with signs of dehydration → call 999/111/GP immediately
• 3–6 months with signs of dehydration → call your GP or 111 the same day
• Any age, severe signs → emergency services immediately

Continue breastfeeding through any illness in young infants. Breastmilk provides fluids and immune support. Do not stop breastfeeding during a vomiting or diarrhoea illness unless advised by a doctor.²

What not to give

• Do not give plain water as the main rehydration fluid for babies under 6 months — it dilutes their blood sodium and can cause hyponatraemia (dangerous low sodium levels)
• Do not give sports drinks (Lucozade, Gatorade) — the electrolyte balance is wrong for babies
• Do not give fruit juice — high sugar content makes diarrhoea worse
• Do not give homemade salt/sugar solutions

If your GP recommends ORS, use the commercial version at the recommended dose for your baby's weight.

Related reading

Dehydration most commonly occurs alongside illness — see Hand, foot and mouth, RSV and other common baby viruses for how these illnesses progress and when to seek care.

← Back to the complete guide: Baby health basics

Also in this cluster: When to call the doctor · Common baby viruses

Sources

1. NHS. "Dehydration." NHS, 2023. https://www.nhs.uk/conditions/dehydration/
2. American Academy of Pediatrics (AAP) / HealthyChildren.org. "Dehydration." HealthyChildren.org, 2023. https://www.healthychildren.org/English/health-issues/conditions/abdominal/Pages/Dehydration.aspx