Mucus, blood, or watery poop in babies: when to worry

You see it: a streak of red. A glob of mucus. Something watery that soaked through. Your stomach drops.

Most of the time, what you're looking at is benign. Sometimes it isn't. This guide walks through the three things parents most often see and panic about — and tells you which version warrants a phone call, a same-day visit, or the ER.

Mucus in baby poop

Mucus shows up as stringy, slimy, sometimes glistening streaks through the stool — like phlegm in the nappy.

The harmless cases

• Teething. Babies swallow extra saliva when teething, and some of it comes through. Common, harmless, transient.
• A passing cold. Swallowed nasal mucus ends up in the nappy.
• Drool, in general. From around 3–4 months, when babies drool nonstop.

A small amount of mucus, with an otherwise normal-looking baby, isn't an emergency.

When mucus matters

• Lots of mucus over multiple days, especially with diarrhoea
• Mucus + blood together
• Mucus + fever, lethargy, or fewer wet nappies
• Persistent mucus + slow weight gain or eczema — sometimes a sign of cow's milk protein allergy in a young baby

These warrant a call to your pediatrician.¹

Blood in baby poop

This is the one that scares parents most. There are several causes — most are mild, but a few are serious.

Common, usually mild

• Anal fissures. A small tear in the skin around the anus from passing a hard stool. Looks like bright red streaks on the surface of the poop or on the wipe. Goes away once the constipation does.
• Cracked nipples (breastfeeding). Swallowed maternal blood can come out the other end as dark or rust-coloured flecks in the first weeks. Look at your nipples — if they're cracked, that's likely the source.
• Cow's milk protein allergy (CMPA). Can cause persistent streaks of blood and mucus in babies under 6 months, often paired with eczema, fussiness, or reflux. Worth a pediatrician conversation, but not an emergency.²

Always call the same day

• Blood mixed throughout the stool (not just on the surface)
• Larger amounts of blood, dark red or maroon
• Blood + persistent diarrhoea
• Blood + fever or vomiting

Go to the ER

• Currant-jelly stools — dark red, jelly-textured, often with mucus. This is a specific sign of intussusception, where part of the intestine telescopes into another. It's a surgical emergency in young children, especially babies between 3 months and 3 years.³
• Large amounts of bright red blood
• Black, tarry stools (after the meconium phase, with no iron supplement) — can indicate digested blood from higher in the gut
• Blood + a baby who is pale, listless, or hard to rouse

Watery poop / diarrhoea

The hard part: breastfed baby poop is already loose and runny. So how do you tell normal from diarrhoea?

The answer is change, plus volume, plus context.

Probably not diarrhoea

• A breastfed baby with their usual seedy, runny stools
• A one-off looser stool with no other symptoms
• A change after starting a new food (often a 24-hour adjustment)

Probably diarrhoea

• A clear change from your baby's normal pattern
• Stools that are more watery, more frequent, and bigger volume than usual
• Often smellier or differently-coloured than baseline
• The nappy soaks through or leaks

When diarrhoea matters

• Lasting more than 24 hours in a baby under 3 months
• Lasting more than 2–3 days at any age
• With fewer wet nappies, fewer tears, dry mouth, or sunken fontanelle (signs of dehydration)
• With fever, blood, or persistent vomiting
• Baby seems unusually sleepy, floppy, or uninterested in feeding

Dehydration is the real risk with diarrhoea, and it can happen quickly in young babies.¹

A quick triage table

When to call your pediatrician

Call the same day for:

• Blood mixed throughout the stool, or more than a streak
• Persistent diarrhoea (>24h in young babies, >2–3 days otherwise)
• Diarrhoea with any sign of dehydration
• Persistent mucus and blood together
• Black, tarry stools with no iron supplement

Go to the ER for:

• Currant-jelly stools
• Large amounts of bright red blood
• A baby who is unusually pale, listless, or unresponsive

Track exactly what you saw

In the moment, photograph the nappy and log what you saw — colour, mucus, blood, watery vs formed. It feels weird but it's the most useful single thing you can hand your pediatrician. Photos and a clear log beat trying to describe a nappy under stress.

← Back to the complete guide: Baby poop overview

Also in this cluster: Baby poop colour chart · Dehydration signs in babies · When to call the doctor

Sources

1. NHS. "Diarrhoea and vomiting in children." NHS.uk, 2024. https://www.nhs.uk/conditions/diarrhoea-and-vomiting/
2. NHS. "Cow's milk allergy in babies." NHS.uk, 2024. https://www.nhs.uk/conditions/baby/weaning-and-feeding/cows-milk-allergy/
3. American Academy of Pediatrics. "Intussusception." HealthyChildren.org, 2023. https://www.healthychildren.org/English/health-issues/conditions/abdominal/Pages/Intussusception.aspx