Cow's Milk Protein Allergy in Breastfed Babies (CMPA/MSPI)

Finding blood or mucus in your baby’s diaper is frightening, especially when you’re exclusively breastfeeding and wondering how anything you do could be the cause. One possible reason is cow’s milk protein allergy (CMPA), sometimes called milk soy protein intolerance (MSPI) in the US. Small amounts of protein from the dairy you eat pass into your milk, and a few babies react to it.

The reassuring part: in breastfed babies this is usually mild, it’s treatable without giving up breastfeeding, and most babies grow out of it. Here’s what to look for, how a dairy-free trial works and when to call the doctor.

What is CMPA in a breastfed baby?

CMPA is an abnormal immune reaction to proteins in cow’s milk, according to HealthyChildren.org from the American Academy of Pediatrics (AAP). In exclusively breastfed babies it often shows up in the gut as allergic proctocolitis: inflammation of the lower part of the bowel that causes blood in the stool. Eczema is the other common sign, according to ABM.

The Academy of Breastfeeding Medicine (ABM) Protocol #24 notes:

• Roughly 0.5–1% of exclusively breastfed babies react to cow’s milk proteins in their mother’s milk. (An older AAP breastfeeding guide puts allergic reactions of any kind at 2–3 in 100 exclusively breastfed babies.)
• Cow’s milk is the trigger in about 50–65% of cases. In one series of 95 breastfed babies with bloody stools, 65% were linked to cow’s milk, 19% to egg, 6% to corn and 3% to soy.
• Babies who are breastfed appear to react to cow’s milk protein less often than babies fed cow’s milk–based formula.

Signs to look for

ABM describes the typical baby as “well appearing” apart from blood in the stool. Common signs, from ABM and HealthyChildren.org:

• Streaks or flecks of blood, or mucus, in the stool, usually starting between 2 and 6 weeks old, although some babies show signs earlier
• Eczema or other skin rashes
• Pain with feeds, such as arching the back or fussing
• Vomiting
• Poor weight gain (uncommon, but important)

Blood loss is usually small, but ABM notes it can occasionally cause anemia, and in very rare cases poor growth. Heavy vomiting, dramatic diarrhea or a swollen belly are unusual with proctocolitis and may point to a different problem.

The NHS adds that cow’s milk allergy symptoms can take up to 3 days to appear, which is part of why it’s hard to link symptoms to a specific meal.

Other causes of blood in the stool

Not all blood means allergy. HealthyChildren.org notes that constipation can cause bright red blood on the stool or when wiping, and ABM recommends checking for a small tear (anal fissure). That’s why the first step is always a visit to your pediatrician, not a diet change. Our baby poop color chart can help you describe what you’re seeing, and if your milk itself looks pink or red, read about breast milk color.

How diagnosis works

There’s no single test. HealthyChildren.org says milk protein allergy is diagnosed from your baby’s symptoms, and ABM notes lab tests are usually unnecessary, although your doctor may check blood counts if there’s a lot of bleeding.

Your pediatrician will usually:

• Ask about family history of allergy and whether your baby gets only breast milk
• Examine your baby and check weight, length and growth
• Look for eczema, a rash or an anal fissure
• Decide with you whether to try a dairy-free trial or wait and watch

The dairy elimination trial

If CMPA seems likely, the main treatment for a breastfed baby is changing your diet, not stopping breastfeeding. ABM Protocol #24 recommends:

1. Cut out cow’s milk protein completely. That means milk, cheese, butter, yogurt, ice cream and anything made with them. Read labels for hidden milk ingredients such as casein, whey and lactoglobulin.
2. Keep going for at least 2 weeks and up to 4 weeks. ABM says most babies improve within 72–96 hours, but some take 2–4 weeks. HealthyChildren.org notes it can take up to 2–3 weeks for the protein to clear from your milk.
3. Keep breastfeeding. For mild to moderate cases, ABM says the baby should continue breastfeeding through the trial.
4. Test whether it was really dairy. If your baby improves, ABM suggests reintroducing dairy to your diet. If symptoms come back, cow’s milk is the likely cause.

Some doctors ask you to cut out soy as well, because babies with CMPA can react to soy protein too (HealthyChildren.org). If cutting out dairy doesn’t help, ABM suggests eliminating other common triggers one at a time. Very restrictive diets should be planned with a registered dietitian, because they can leave you short of nutrients.

Our guide to foods to avoid while breastfeeding explains why most parents don’t need to restrict their diet at all, and why CMPA is an exception.

“Watch and wait” is sometimes an option

Expert guidance has shifted somewhat. A 2024 review of international guidelines in Nutrients found that for breastfed babies with mild bleeding who are otherwise well, several groups consider a 2–4 week “watch and wait” approach reasonable, partly because bleeding stops without any diet change in some babies (about 20% in one study). The authors also flag concerns about overdiagnosis and about strict diets shortening breastfeeding. Ask your pediatrician which approach fits your baby.

Looking after yourself on a dairy-free diet

Dairy is a big source of calcium. For mothers on an elimination diet, ABM Protocol #24 recommends continuing your daily vitamins and adding calcium, 1,000 mg a day split into several doses. Check the product and dose with your doctor, pharmacist or dietitian.

Quick dairy-free fuel matters when you’re feeding around the clock. See our breastfeeding snack ideas and choose the dairy-free options.

How long does it last?

• ABM advises that if dairy is confirmed as the trigger, you avoid it until your baby is 9–12 months old and for at least 6 months after diagnosis.
• HealthyChildren.org says most infants outgrow cow’s milk allergy by age one, and dairy can be slowly reintroduced after 1 year with your pediatrician’s guidance.
• ABM notes that babies with a history of allergic proctocolitis don’t need to avoid other major allergens when starting solids, unless a specific allergy has been identified.

When to call the doctor

Call your pediatrician if you see any blood in your baby’s stool, even a small streak. Get in touch promptly if:

• Your baby isn’t gaining weight, or is losing weight
• There is more than a small amount of blood, or it keeps happening
• Your baby is vomiting repeatedly, has a swollen or tender belly, or has diarrhea
• Your baby seems unwell, very sleepy or feeds poorly
• Symptoms don’t improve after 2–4 weeks of a dairy-free diet

ABM recommends referral to a pediatric allergist or gastroenterologist for babies with poor growth, larger amounts of blood with falling hemoglobin, or protein loss.

Call 911 if your baby has signs of a severe allergic reaction (anaphylaxis), which the NHS describes as breathing problems, a swollen throat or tongue, or a raised itchy rash that comes on quickly.

A dairy-free trial can feel overwhelming on top of a new baby. An IBCLC or dietitian can help you keep breastfeeding and eating well. Find support on our breastfeeding support page, or browse more guides in our Common challenges hub.