CMPA Baby Poo: What Should Parents Look For?

If you’ve opened your baby’s nappy and found green poo, mucus, very loose poo or even a little blood, it’s completely understandable to wonder whether cow’s milk protein allergy (CMPA) could be causing it.

I get asked about baby poo a lot.

Parents will often say to me:

“Does this look like CMPA poo?”

And the tricky answer is that there isn’t one particular type of poo that tells us a baby definitely has a milk allergy.

CMPA can affect your baby’s tummy and their poo, but I would always want to know what else is going on too.

So let’s look at some of the poo changes you might see, what they could mean and when I’d recommend getting some individual advice.

What can CMPA baby poo look like?

There isn’t one classic “CMPA poo”.

Babies with cow’s milk allergy can have:

• Loose or very frequent poo
• Diarrhoea
• Mucus in their poo
• Blood in their poo
• Constipation
• Lots of straining
• Tummy discomfort
• Painful wind

These are all recognised symptoms that can occur with delayed food allergy. NICE includes loose or frequent stools, blood and/or mucus, constipation and tummy pain amongst the possible gastrointestinal symptoms of non-IgE-mediated food allergy.

But — and this is really important — all of these things can happen for reasons other than milk allergy too.

That’s why I wouldn’t look at one nappy and say, “Yes, that’s CMPA.”

I’d want to know what your baby is like overall.

Does mucus in baby poo mean CMPA?

Mucus can be one of the symptoms we see with cow’s milk allergy.

It can look slimy, stringy or jelly-like and you might notice it mixed through the poo.

But a bit of mucus does not automatically mean your baby has CMPA.

Allergy UK also points out that small amounts of mucus can sometimes be seen when babies have things like colds or are teething.

This is why context matters.

If you’re seeing mucus alongside other symptoms such as:

• Reflux or vomiting
• Significant discomfort
• Blood in the poo
• Eczema
• Feeding difficulties
• Poor weight gain

then I’d be much more interested in looking at whether there could be an underlying milk allergy.

Mucus on its own gives us much less information.

My baby’s poo is green – is that CMPA?

Not necessarily.

I see parents worrying about green poo all the time, but green poo by itself is not a good way of diagnosing cow’s milk allergy.

Baby poo comes in all sorts of colours.

And if your baby has already been diagnosed with CMPA and has started a hypoallergenic formula, their poo can actually become darker or greener as a result of the formula.

That can be completely normal.

So if your baby has green poo but they’re happy, feeding well and growing normally, I wouldn’t immediately assume milk allergy.

Again, I’d look at what else is happening.

What about blood in baby poo?

Blood can occur with cow’s milk allergy, particularly with some delayed gut reactions.

You might see a small streak or little flecks of blood within the poo.

But blood can have other causes too, so I wouldn’t automatically assume it is CMPA.

For example, babies can get tiny anal fissures, particularly if they’ve been straining or passing harder poo.

If you keep seeing blood in your baby’s nappy, or your baby is also unwell, vomiting, feeding poorly or seems in pain, I would get medical advice rather than trying to work it out from photos online.

Can CMPA cause really loose poo?

Yes, it can.

Some babies with cow’s milk allergy have very loose or frequent poo.

But breastfed baby poo can naturally be pretty loose anyway, which can make this one difficult for parents to interpret.

Rather than looking at whether the poo is simply “loose”, I’d ask:

• Is this different from your baby’s usual poo?
• Has it changed suddenly?
• Is it happening repeatedly?
• Is there mucus or blood?
• Is your baby uncomfortable?
• Are there symptoms elsewhere, such as their skin or feeding?
• Are they growing as expected?

That gives us much more useful information than one messy nappy.

Can milk allergy cause constipation too?

Yes, which often surprises parents and some health professionals!

We tend to think of milk allergy causing diarrhoea, but some babies actually have constipation or a lot of straining instead.

NICE includes constipation as one of the possible gut symptoms seen with non-IgE-mediated food allergy.

But constipation is also really common in babies and children.

So, just like mucus or loose poo, constipation on its own doesn’t diagnose CMPA.

What can CMPA poo look like in a breastfed baby?

This can be especially confusing because breastfed babies can have such variable poo anyway.

It might be yellow one day, slightly green another day, very loose, seedy or completely different from the day before.

A breastfed baby with CMPA might have mucus or blood in their poo, or you may notice other symptoms such as reflux, discomfort or eczema.

But I would be very cautious about telling a breastfeeding mum to remove dairy from her diet based on poo alone.

Cow’s milk proteins from mum’s diet can pass into breast milk in very small amounts, so CMPA can occur in an exclusively breastfed baby. But we still need to look at the whole clinical picture. CMPA is less common in exclusively breastfed babies than in babies receiving cow’s milk-based formula.

If you’re considering cutting dairy from your diet because you think your breastfed baby has CMPA, I’d recommend getting some advice first.

Removing dairy unnecessarily could affect important nutrients such as calcium and iodine if they aren’t replaced properly.

What about formula-fed babies?

Formula- fed babies can also have quite a bit of variation in their poo.

If CMPA is suspected, you may see things such as mucus, blood, loose poo, constipation or straining.

But there’s another important thing to know.

If your baby is changed onto an extensively hydrolysed or amino acid formula, their poo can look completely different afterwards.

It might become:

• Greener
• Darker
• Smell different
• Change in frequency

That doesn’t necessarily mean the new formula isn’t agreeing with them.

It can simply be the way their poo looks on that particular formula and can take some time to settle.

Does smelly baby poo mean milk allergy?

No, not on its own.

Poo can smell stronger for all sorts of reasons, feeding changes, formula, weaning foods, illness and just normal changes in the gut.

So if the only thing you’ve noticed is that your baby’s poo smells different, I wouldn’t use that as evidence of CMPA.

I’ve actually written a separate guide about baby poo that smells like vinegar, because this is another thing parents search for a lot.

This is why I don’t diagnose CMPA from a nappy picture

I know it can be tempting to compare your baby’s nappy to pictures online.

But reflux, eczema, colic, loose poo, constipation, mucus and unsettled behaviour are all incredibly common in babies.

That means we need to look at patterns, not individual symptoms.

If I was assessing your baby, I’d want to know things like:

• What are they feeding on?
• When did the symptoms start?
• What happens after feeds?
• Do they have eczema?
• Are they vomiting or refluxing?
• Is there mucus or blood in the poo?
• Are they feeding happily?
• How are they growing?
• Have symptoms changed with any dietary changes already made?

That gives us a much clearer picture.

For delayed non-IgE cow’s milk allergy, allergy blood tests and skin prick testing aren’t generally how we confirm the diagnosis.

A planned period of milk-protein exclusion followed by reintroduction is usually needed to see whether symptoms improve and then return.

That’s also why randomly removing milk for a few days and deciding it has “worked” can sometimes make things even more confusing.

What other CMPA symptoms should I look for?

If your baby’s poo has changed, have a think about what else you’ve noticed.

CMPA can also cause symptoms such as:

• Reflux or vomiting
• Eczema or worsening eczema
• Hives or swelling
• Feeding difficulties or refusing feeds
• Lots of discomfort or painful wind
• Poor weight gain
• Coughing or wheezing with an immediate reaction

Some babies mainly have tummy symptoms. Others have symptoms affecting their skin too.

You can read more about this in my full guide to

Should I cut out dairy because my baby has mucus or green poo?

I wouldn’t recommend doing that based on one symptom alone.

I’ve seen lots of parents end up removing dairy, sometimes alongside soya and other foods — because they are desperately trying to work out what’s causing their baby’s symptoms.

Before long their own diet, or their baby’s diet, can become quite restricted and they’re still not sure whether milk was actually the problem in the first place.

If CMPA is suspected, the aim should be to carry out a proper assessment and have a clear plan.

That is particularly important if:

• You’re breastfeeding and thinking of removing dairy yourself
• You’re considering changing your baby’s formula
• Your baby is already avoiding several foods
• Feeding has become difficult
• You’re worried about growth

We want to make sure we’re not missing a genuine allergy — but we also don’t want to remove foods unnecessarily.

How quickly should my baby’s poo improve on a milk-free diet?

This is another question I get asked all the time.

Parents understandably expect that if milk is the problem, they should see a completely normal nappy within a day or two.

It doesn’t always work like that.

Gut symptoms can take time to settle, and Allergy UK notes that stool symptoms and mucus may take several weeks to improve.

This is why I find it much more useful to look at trends over time.

Has the mucus reduced?

Is your baby more comfortable?

Are they feeding better?

Has their skin improved?

Are they growing?

Those changes can be far more useful than trying to decide whether today’s nappy looks slightly better than yesterday’s.

When should I get medical advice?

Get medical advice if you are seeing things such as:

• Persistent blood in your baby’s poo
• Repeated vomiting
• Significant diarrhoea
• Poor feeding
• Concerns about growth
• Signs of dehydration
• Significant or worsening eczema alongside gut symptoms
• Repeated symptoms following milk exposure

If your baby develops breathing difficulties, significant swelling, becomes pale or floppy, or you think they may be having a severe allergic reaction, call 999/911.

Still wondering whether your baby’s poo could be due to CMPA?

This is often the point where Google stops being very useful.

You might have read ten different articles and looked at twenty different nappy pictures and still be thinking:

“Is this actually CMPA?”

“Is that amount of mucus normal?”

“Should I remove dairy?”

“Do I need to change the formula?”

“Have I given the milk-free diet long enough?”

And those aren’t questions I can answer properly from the appearance of poo alone.

I need to look at your baby — their feeding, symptoms, growth, skin, reflux, nappies and what you’ve already tried.

If you’re going round in circles trying to work out whether your baby’s symptoms fit with cow’s milk allergy, this is exactly the sort of thing I can help you with in a consultation.

You might also find these useful:

CMPA SYMPTOMS IN BABIES

CMPA FORMULA; HYPOALLERGENIC FORMULA OPTIONS

MILK ALLERGY VS LACTOSE INTOLERANCE IN BABIES

The main thing to remember

There isn’t one type of CMPA baby poo.

Mucus, green poo, blood, diarrhoea and constipation can all occur in babies with cow’s milk allergy, but they can also happen for other reasons.

So try not to analyse every nappy in isolation.

Look at your baby as a whole.

And if you’re still not sure what you’re seeing, that’s usually much more useful to discuss with someone who can look at the full picture than trying to match your baby’s poo to another photo online.