Baby poop can vary greatly. If poop is more watery and frequent than usual and the baby has other symptoms, such as vomiting or refusing to feed, a parent or caregiver may need to consult a doctor. A cow’s milk allergy may be the cause.

Cow’s milk allergy is the most common food allergy in infants. It affects up to 3% of babies in developed countries by the time they turn 1 year of age.

This article discusses milk allergy in babies, how it affects their poop, other symptoms, and how to manage it.

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Typical baby poop varies in appearance according to what the baby eats and their age.

When babies are newborn, their poop is thick and black. This is called meconium, which consists of amniotic fluid, skin cells, and other substances the fetus swallowed while in the uterus.

After the first few days, a baby’s poop will transition. The transitional stool will be more of a yellowish-green color and not as thick as meconium.

Once the baby is a few days older, their poop will depend largely on whether they drink breastmilk or formula.

A typical poop from a breastfed baby is usually yellow, green, or brown and seedy. Their stool may be quite soft or runny, but that does not necessarily mean they have diarrhea.

Babies who drink breast milk often need more nappy changes than formula-fed babies, as they digest breast milk faster.

A typical poop from a formula-fed baby is often thicker than that of a breast milk-fed baby. It will be yellow or brown and similar to the texture of peanut butter.

After around 6 months, babies begin eating solid foods. At this time, their poop often becomes firmer and the color may reflect what they eat. For example, babies who eat carrots, sweet potatoes, and pumpkin may have orange poop.

Learn more about baby poop color.

The most common sign that a baby is allergic to milk is blood in their stool. A baby’s poop may also become more runny and frequent, similar to diarrhea. It may also contain mucus.

Learn more about blood in a baby’s stool.

A milk allergy, sometimes called a cow’s milk protein allergy, occurs when the baby’s immune system overreacts to milk protein. It generally occurs with cow’s milk, but it can also occur with sheep and goat milk.

Although babies younger than 6 months do not usually eat solid foods, they may react to dairy proteins in formula or breast milk.

When this happens, the baby’s body will produce histamines to “attack” the milk. It is different from lactose intolerance, which occurs when a person can not digest the sugar in the milk, called lactose.

About 80% of children will outgrow the allergy by the time they reach 16 years.

The allergic reaction can cause various symptoms aside from a change in the baby’s stools.

Like other food allergies, symptoms of a cow’s milk allergy can range from mild to severe. These symptoms can include:

  • hives or a red, itchy rash — though there may be no redness on darker skin
  • upset stomach
  • vomiting
  • colic
  • coughing
  • wheezing
  • congestion

Symptoms often start within 2 hours of coming into contact with milk but can also be immediate. They may also appear many hours later.

Delayed reactions

Most reactions to milk appear soon after a baby comes into contact with milk. However, in some, a reaction can occur more than 20 hours after contact. Doctors call this delayed type hypersensitivity.

Symptoms of these delayed reactions are mainly gastrointestinal. A parent or caregiver may notice:

  • changes in stool
  • bloody diarrhea
  • weight loss
  • vomiting
  • pale skin
  • lethargy

Serious reactions

In rare cases, a baby may have a serious allergic reaction to milk, resulting in anaphylaxis, which can be life threatening.

Symptoms of a serious reaction include:

  • difficulty breathing
  • swelling of the tongue
  • shock
  • drop in blood pressure

Infants may also experience nonclassical signs of anaphylaxis, including:

  • ear pulling
  • tongue thrusting
  • fussiness
  • increased clinginess to the caregiver

Serious reactions generally occur within minutes of consuming milk or foods containing milk.

If they suspect a milk allergy, caregivers can take a baby to a pediatrician for a diagnosis. The pediatrician may recommend an appointment with an allergist.

An allergist will conduct a medical review and discuss the baby’s symptoms, including when they occur and their severity. They may also ask about the parents’ medical history.

During the exam, the doctor may recommend using a skin prick test. In this test, the doctor makes a scratch with a small milk sample on the skin. They will then wait 15–20 minutes to see if a rash or welt appears. They will likely diagnose the baby with a milk allergy if it does.

However, around half of milk allergies in babies are “nonimmunoglobulin E (IgE)-mediated.” This means they occur from a reaction involving components of the immune system other than IgE antibodies. These allergies typically take longer to present with symptoms, making them difficult to recognize. Skin testing does not help in diagnosis. In these cases, an allergist will recommend excluding milk from the diet and then reintroducing it to see if a reaction occurs. This will help confirm a cow’s milk allergy.

Learn more about allergy testing.

Unless the baby experiences a severe, life threatening reaction, the best treatment for a milk allergy is to avoid milk and foods containing it.

If the baby drinks formula, a doctor will recommend switching to a soy or dairy-free formula option.

Breastfed babies can react to milk proteins if the person producing breast milk consumes dairy. In these cases, the nursing individual should avoid dairy products.

For children with a food allergy, doctors may advise caregivers to carry an auto-injector containing epinephrine, which treats anaphylactic shock.

Learn about dairy milk substitutes.

At around 6 months, babies begin trying solid foods, although their main source of nutrition remains either formula or breast milk.

When introducing new foods, parents and caregivers should pay attention to food labeling. It is best to avoid all products that contain milk or dairy.

Labels may help people avoid potential exposure, but a person can also benefit from knowing which foods to avoid. Some products that contain dairy include:

  • butter
  • yogurt
  • custard
  • pudding
  • milk
  • some baked goods
  • cheese
  • cream
  • ice cream

If in doubt, a parent and caregiver can talk with a health professional about foods that may cause a reaction in a baby.

A parent or caregiver should contact a doctor immediately if they notice blood in a baby’s poop. This is the most common sign of a milk allergy.

A caregiver may also want to contact a doctor if they notice any unusual digestive symptoms in their baby. A doctor can help diagnose the issue.

Babies with a milk allergy may outgrow it as they get older. About 80% of children will outgrow the allergy when they are 16 years of age.

Children who have high levels of cow’s milk antibodies in their blood are the least likely to outgrow their allergy. An allergist can use blood tests to measure a child’s antibodies.

Those who do not outgrow the allergy usually need to avoid milk and dairy products. However, allergy experts suggest that consuming dairy in baked goods may help build up a tolerance. Caregivers should talk about this with an allergist before trying it at home.

Babies can react to milk proteins in formula, breast milk, or solid foods. The most common sign is blood in the stool.

Allergic reactions range from mild to severe. Mild reactions cause symptoms such as changes in the stool and vomiting. A severe reaction can cause difficulty breathing and may be life threatening. In these cases, people should seek immediate emergency help.

Most babies eventually outgrow a milk allergy. Until they do, parents and caregivers should avoid giving them dairy products. They may also need to keep an emergency epinephrine auto-injector in case of accidental exposure and a severe reaction.