Don’t you sometimes wish that your newborn came with an instruction manual? It would take the guesswork out of some of those pressing questions of the first few weeks: Are they eating enough? Crying too much? Too hot? Too cold? Why are they spitting up when I feed them?

Let’s focus on that last one for a minute. It’s important to remember that for the first few months, spitting up is very normal for babies. And even if the consistency of what comes out seems a little different than what went in, it’s still generally nothing to worry about.

Here’s why your baby might be spitting up curdled milk — and what you can do about it.

Babies spit up all the time because of their undeveloped digestive system. It happens when milk or food comes back up the baby’s esophagus from the stomach. (The esophagus is the organ connecting the stomach to the mouth).

Spitting up and vomiting are separate actions. Vomiting requires force and can be painful in babies. When it comes to spitting up, however, it’s likely your little one won’t even notice when they’re doing it.

Normal spit up for breastfed and formula-fed babies will usually look quite similar to the milk or formula that the baby just had — more about the curdling in a minute. Your baby will be relatively comfortable and content during and after spitting up — they may even look pleased with themselves! If they’re not done eating, they’ll have no problem accepting more food after a spitting session.

Spit up, just like vomit, can contain stomach acid. Babies’ spit up becomes curdled when milk from breastfeeding or formula mixes with the acidic stomach fluid.

Time also plays a role here. Immediate spit up after feeding will probably look like regular milk. If your little one spits up after some time as passed, it’s more likely to look curdled milk.

There are a few possible reasons why your baby may be spitting up curdled milk:

Difficulty with digestion

Your baby may have trouble digesting breast milk for a few reasons. The most common — at least for newborns — is just that their digestive system needs more time to develop.

Some babies may have an allergic reaction to cow’s milk or formula that contains cow’s milk. Rarely, they may have lactose intolerance or a milk protein allergy. But keep in mind that these come with other symptoms, like diarrhea. Talk to your pediatrician if you suspect an allergy or intolerance.

Your pediatrician may recommend a shift in your baby’s diet to reduce spit up. Options include:

Keep in mind, though, that babies typically grow out of digestive problems with age.

Acid reflux

Babies may have acid reflux, especially if they’re newborns. Acid reflux happens when stomach acid travels up the throat from the stomach. It can be painful.

Before switching formulas (or switching from breastfeeding to formula), try these home remedies:

  • avoid feeding your baby too fast or too much at once
  • feed your baby in an upright position.
  • if you’re breastfeeding, avoid foods like citrus, tomatoes, spicy foods, carbonated drinks, and foods high in fat (these can affect your breast milk and lead to reflux)

Pyloric stenosis

Pyloric stenosis is rare, affecting about 3 out of 1,000 babies in the United States. The pylorus is the lower part of the stomach that food and other substances pass before entering the small intestine.

The muscles in the pylorus may become enlarged in infants, which causes the pyloric channel to narrow and prevents food from leaving the stomach. Most babies that experience pyloric stenosis are 3–5 weeks old and require treatment and surgery.

Some signs of pyloric stenosis include projectile vomiting of milk, weight loss, and constipation. Pyloric stenosis is a serious condition that requires immediate treatment.

There are several reasons that may lead to your baby spitting up stomach acid. Spit up may appear curdled if your baby hasn’t digested milk from feeding. This isn’t always a health concern, but it’s important to pay attention to any changes in your baby’s spit up.

Talk to your baby’s doctor if you are concerned about your baby’s spit up or have questions about healthy feeding alternatives.