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Why Baby Poop Smells Like Vinegar (Plus Other Strange Diaper Smells!)

Why Baby Poop Smells Like Vinegar (Plus Other Strange Diaper Smells!)

Medically reviewed by

Dr. Nicole Nabatkhorian
Dr. Nicole Nabatkhorian

MD / PGY-2 Pediatrics

Disclosure: Some of our articles contain links to recommended products or services in which we may receive a commission if you make a purchase.

After you’ve changed your baby’s diaper enough times, you’ve probably noticed a few odors that you weren’t expecting. Most of the time, this isn’t a cause for concern. However, when you get a whiff of a particularly strange smell, like when baby poop smells like vinegar, you might wonder if this is normal.

When your baby’s poop smells like vinegar or another strange smell, this is likely due to a food sensitivity or allergy (especially dairy), a virus or bacterial infection, or mild dehydration. Sweeter smells are usually a good sign and are common in newborns, especially breastfed babies.

In my experience, there are plenty of strange smells that can come from your baby’s diaper. In most cases, these smells go away on their own or change to something new when you introduce new foods. Let’s dive into what might be causing the strange odors coming from your baby’s diaper.

Baby in diaper laying on bed

Baby poop smells like vinegar: Why it happens

During the first couple years of your baby’s life, it’s common to experience many different scents, colors, consistencies, and other abnormal issues with your baby’s poop and pee. You’ll notice consistent changes as their bodies develop and adjust to digesting and absorbing different kinds of foods.

For your peace of mind, it’s important to know that most changes to color, consistency, and odor are a completely normal part of a baby growing up and they will typically go away or normalize over time.

However, there are some things you need to look out for. Anytime your baby’s stool is runny, dark red or black, mucusy, or white it could be a sign of a more serious issue. If you notice any of these changes in your baby’s stool, consult with their doctor right away.

This is also the case if strange poop symptoms are happening alongside other issues like fever, vomiting, etc. In most cases, it’s still an easily treatable condition, but these are the times to get expert medical advice!

By the way, I’ve written an awesome article all about what to do if your baby’s room (or any other room in the house) smells like poop. Check it out here for some great advice!

Let’s take a closer look at why your baby’s diaper might smell like vinegar.

Why does my baby’s poop smell like vinegar?

When baby poop smells like vinegar, you might become concerned and wonder if something is wrong. There are a handful of reasons why your baby’s diaper smells this way. Let’s take a look at some of the possible causes.

Food allergy or sensitivity

No matter what age your baby is, food allergies and sensitivities are always something to look out for. Especially when you’re first introducing new foods to your little one, it’s important to pay attention to how they react to each new addition. A food allergy or sensitivity could lead to a vinegar-smelling poop in a formula fed or breastfed baby.

Dairy or lactose is the most common food allergy. If your baby is exclusively breast-fed, they may be allergic to something you’re eating or drinking that is passed on through your milk.

Nutrient malabsorption

Nutrient malabsorption occurs when the food your baby eats aren’t properly absorbed by their body. Some of the undigested food or milk in their poop can lead to a sour or vinegar smell.

When food isn’t properly digested, this might be due to a specific disorder or an imbalance in the digestive system. If your baby has an issue with malabsorption, they may experience other symptoms such as stomach pain, gas, diarrhea, or bloating.

Gut microflora imbalance

The proper amount of gut flora in the digestive system is a necessary part of a healthy digestive tract. When there is an imbalance in the gut microflora, this can lead to digestive issues, which may cause a sour or vinegar smell in your baby’s diaper.

Like malabsorption, this would likely come with other symptoms such as cramps, bloating, or constipation.

Reaction to medication

Since your baby’s digestive system is still developing, you might notice a reaction to any changes, such as your baby taking a new medication. If your baby is on antibiotics, for example, this type of medication is notorious for causing tummy issues in little ones.

If your baby recently started a new medication, this could be the cause of their sour-smelling poop.

Crohn’s disease

Although this is less likely than some of the other causes, Crohn’s disease causes the intestinal wall to become inflamed, which could lead to sour-smelling baby poop.

Some of the most common symptoms of Crohn’s disease are diarrhea, stomach pain, fever, and excessive weight loss.

Parent feeding baby in booster seat

Change in baby’s diet

With a baby’s sensitive digestive system, any change in the food they’re eating can impact their poop and cause temporary tummy issues. Even when there isn’t a food allergy present, it may take time for them to adjust to something new.

If your baby’s poop smells sour, you might consider if you’ve introduced any new foods or anything new into their meal routine.


Teething is the cause of many frustrations, especially when you have a little one who has multiple teeth coming in at once. Since teething babies drool so much, the excessive amount of saliva can cause changes to their stool.

If your baby is teething, you will typically notice excessive drooling and your baby may want to chew on everything to ease the pain.


Rotavirus is a contagious stomach virus that babies are typically vaccinated for at 2 months, 4 months, and 6 months. Babies with rotavirus usually experience symptoms such as fever, vomiting, and diarrhea.

If your baby has rotavirus or a similar illness, they may have foul-smelling poop, as well as watery diarrhea.

Parent changing their baby's diaper

What if the baby poop smells sour?

An odor of vinegar or the smell of sour milk is a pretty common one to find when changing a diaper and there are a few reasons why you might detect the acidity.

Lactose overload is another one of the common culprits for vinegar-smelling diapers. Lactose is found in both cow’s milk and breast milk at about the same concentration, seven percent.

The issues arise when the baby can’t produce enough lactase to keep up with the amount of lactose being fed to them either through bottle-feeding or breastfeeding, usually because of overfeeding.

With breastfed babies, this problem can also surface if the baby is getting more foremilk (the milk that comes out first during a feeding) than hindmilk (the milk that drops later in the second half of a feeding) because there is a lower fat content in this milk and thus a higher percentage of lactose.

You might also notice that the poop isn’t the normal light yellow color, but has darkened up because of the lower fat content. The best way to prevent this particular issue is to block feed or completely empty one breast before moving on to the next.

Is it normal for baby poop to smell sour?

If your baby is showing other symptoms such as green or black stool, blood in the stool, vomiting, fever, etc. then the sour milk smell could be from a virus or bacterial infection. In this case, the smell should resolve after the other issue passes!

A sensitivity or allergy to a new food is another extremely common reason for vinegar or sour milk smell. Babies can have a reaction to new foods as they transition to solids. They can also be exposed to these foods through mother’s breast milk in some cases, especially cow’s milk.

Food allergies can also cause all sorts of other more serious issues like diarrhea, constipation, hives, and vomiting. A quick trip to the doctor, possibly with a dirty diaper, should be able to diagnose a milk allergy quickly.

By the way, I’ve written an article all about baby neck cheese and the sour milk smell it can cause. Check it out here – maybe the odor you smell is from this!

I’ve never experienced this one myself, but many people say that teething is linked to a vinegar smell both in the diaper and on the baby in general. If are having a hard time tracking down this strange scent and have eliminated the other options, check to see if your baby has a tooth about to come out!

What if the baby poop smells like ammonia?

Ammonia is by far one of the most common scents to catch a whiff of in a baby’s wet diaper because it’s actually a normal component of urine.

Ideally, urine won’t actually smell like anything because the kidneys have filtered it through and the body is processing everything in the way it should. If your baby is mildly dehydrated, such as first thing in the morning, then the urine can become more concentrated and show up as a darker yellow and smelling of ammonia.

If the smell is mild, comes and goes, and doesn’t follow any other symptoms, then it is probably not a cause for concern.

What if the baby poop smells like metal?

Although not as common, it could be a little alarming if you detect a metallic odor when you are changing a diaper and there are a couple of reasons why you might find this one.

Dehydration – If your baby is dehydrated, it can raise the relative concentration of minerals and other things that have ended up in the urine, giving it a faintly metallic smell.

Look for other symptoms of dehydration if you suspect this is the cause such as infrequent wet diapers, dark yellow urine, lethargy, listlessness, and a sunken soft spot at the top of your baby’s head. If you see these symptoms, seek medical advice right away.

The diapers themselves – Although I have never experienced this particular issue, many parents have reported that their baby’s diaper itself was the culprit for a metallic smell. They figured this out by wetting the diapers with water and giving them the sniff test.

Sure enough, the absorbent material gave off the odor even with tap water. The diapers mentioned were the regular Huggies and Pampers brands and while it hasn’t been linked to any dangerous outcomes, be sure to check your own supply for this issue as it sounds alarming to me.

Pseudomonas infection – Out of the left field, we have an infection with a particular bacteria that gives urine a metallic scent and serves as a quick identification. You shouldn’t have to worry too much about this one with your baby, however, because it’s almost always found in nursing homes.

What if the baby poop smells like popcorn?

No, you aren’t crazy. Sometimes baby poop will smell like buttered popcorn or even sweet-smelling things like cakes or bread. If you are experiencing this issue and you are breastfeeding, then you probably don’t have anything to worry about.

Breastfed babies can have often have poop with a sweet smell that doesn’t resemble what we normally think of when it comes to poop.

Dad changing baby's diaper on changing table

What if the baby poop smells like maple syrup?

If you are detecting the smell of maple syrup on your baby, especially in the morning, then it could be for the same reason as the buttered popcorn above. Baby poop can often have a sweeter smell in breastfed babies that can actually be quite pleasant.

I separated out this particular smell, however, because there could be a much more serious reason for it happening. There is such a thing as MSUD (Maple Syrup Urine Disease) which is a very dangerous and life-threatening disorder that prevents babies from breaking down proteins into their amino acid parts properly.

Importantly, you probably don’t have to worry about this one. This disorder typically rears its ugly head very early on with other symptoms that are way worse than the smell (weight loss, weak suck, poor appetite, sluggishness, vomiting, and even seizures or coma) and most hospitals and doctors screen for this at birth anyway. I just wanted to mention this one here to put your mind at ease because it’s very rare, but it will pop up in a quick search and scare you!

It’s more likely that you’ll notice this smell once your baby is a little older (6 months-1.5 years) and it could either be the urine or sweat that smells like maple syrup. It is likely the result of mild dehydration overnight, especially if your baby doesn’t wake for feedings, or sweat buildup due to wearing warm clothes. Neither of these is likely a major issue, but check with your doctor on your next visit just in case.

Because older diabetes patients can show this symptom as a result of their body not breaking down sugar properly, parents may also think that this is the culprit for their baby. Diabetes in babies, especially newborns, is exceedingly rare unless it is type 1 diabetes.

In this case, there will likely be more severe symptoms such as drowsiness, intense thirst, heavy breathing, vision changes, weight loss, and others. If your baby has any of these other symptoms, see a doctor immediately.

What if the baby poop smells like yeast?

A yeasty smell, sometimes described by parents as a bread or beer-like scent, is another common odor for a diaper that probably isn’t a cause for concern. Just like the buttered popcorn and maple syrup, baby poop can smell a little yeasty, especially in breastfed babies, because of the way they’re breaking down sugars in the milk or formula.

Some of those sugars might be making their way through the digestive system and contributing to the smell.

The biggest possible cause for the concern is that your baby has some sort of yeast infection brewing and you are smelling the result, but you’ll usually see that rather than smell it. If it is a yeast infection, look for:

  • Bright red patches of inflamed skin, especially in skin creases around private areas and bottoms, under chins, etc.
  • Red bumps around the irritated skin
  • Any kind of diaper rash that won’t go away with standard creams or other remedies

Because yeast infections have to be treated with an antifungal medication and they don’t respond to over-the-counter diaper rash remedies, it will likely require a trip to the pediatrician to get this cleared up.

What if the baby poop smells like onions?

This is a bit of a random one as far as smells go, but I’ve seen many people report this one over the years. There doesn’t appear to be any concerning medical reason that your baby’s pee or poop would smell like onions so it’s probably nothing to be too alarmed about.

The most likely reason for this issue is food-related. If you have been introducing onions to your baby as new solid food, it’s possible that they aren’t digesting it very well and they are passing through the bowels mostly intact.

Once they arrive in the poop, they could still have enough odor left to make the whole thing smell like onions. It’s common to find all sorts of intact solid food in a baby’s stool while their stomach and colon make the transition away from breast milk.

What if the baby poop smells like burnt rubber?

Burnt rubber or any kind of burning plastic smell is a strange odor to find in your baby’s diaper. Unlike the other issues, there doesn’t seem to be a definitive answer as to why this would happen. Several people have reported this smell off and on in baby forums, however, and there a few reported causes discovered.

If all you notice is the smell and it comes and goes with no other symptoms showing in your baby, it’s probably just related to something they ate (if the baby is eating solids) or something you ate (if you are breastfeeding) and likely isn’t serious.

One of the causes ended up being an allergy to milk for many people. Even if your baby is breastfed, they can still have issues from the milk if mommy is drinking it before feedings. The best way to check for this is to eliminate dairy from both of your diets for several days to see if anything changes. Along with milk, it could be the sign of another sensitivity or allergy so it might require a process of elimination to figure out.

Another likely cause is a virus or other ‘stomach bug’ that causes general gastrointestinal upset. If this is the cause, a strange smell will only be one of the symptoms that you’ll see, not the only one. Along with the burnt rubber odor, check for mucus or blood in the stool or green or black stool. In addition, there will likely be other symptoms such as vomiting, diarrhea, fever, etc.

What does normal baby poop smell like?

Blank’s Children’s Hospital has created an amazing resource for parents to discover what normal baby poop should look like during the different stages of development and under a variety of different circumstances. They also have great call outs for odd colors and other things that parents frequently get freaked out about with their babies.

By the way, what about when your baby farts smell but there isn’t any poop to back it up? I’ve got you covered! You can check this infographic here.


There are many different smells that may come from your baby’s diaper. When baby poop smells like vinegar, or something else that you don’t expect, you might be concerned that something is wrong. Most of the time, if there are no other symptoms, it’s likely due to something your baby ate. However, always check with your little one’s doctor if you have any questions or concerns.

Joshua Bartlett
Joshua Bartlett

My name is Joshua Bartlett I run this blog with my wife Jarah. We have more than 11 years of parenting experience including three girls and one boy. I started this blog in late 2018 when I realized that I was dealing with baby-related issues on a constant basis…please read more about me here!