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Why a diaper smells like vinegar, popcorn, yeast, metal

Why Baby Diapers Smell: Vinegar, Metal, Popcorn, and More!

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After you have changed your baby’s diaper a few times you’ve probably noticed a few odors from time to time that you weren’t expecting. Most of the time, this isn’t a cause for concern but you’ve probably caught yourself wondering sometimes why your baby’s diaper smells the way it does.

The reason your baby’s diaper smells like something off or unpleasant is likely due to a sensitivity to a new food, a food 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 personal experience, I’ve smelled all kinds of strange things emanating out of my babies’ diapers over the years. In most cases, these smells went away quickly on their own or changed to something new as we introduced new foods. Let’s dive into what might be causing your strange odors!

Baby diapers can smell like many different things

Baby poop and pee are strange.

During the first couple of years, it is incredibly common to experience many different scents, colors, consistencies, and other abnormal issues with your baby’s poop and pee as their bodies develop and adjusts to digesting and absorbing different kinds of foods.

It is important to know, for your peace of mind, 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.


Anytime that your baby’s stool is runny, dark red or black, mucusy, or white it could be a sign of a more serious issue that you should get checked out quickly. 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!

What if the diaper smells like vinegar or sour milk?

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 one of the most 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. Just because your baby is experiencing a temporary lactose overload doesn’t necessarily mean she is lactose intolerant, but it could point to this problem as well if it doesn’t resolve over time.

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.


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 they are exposed to 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 issues that are more serious like diarrhea, constipation, hives, itchy noses, 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 and I wasn’t able to find any link to a scientific reason why this would happen, 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 diaper 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 diaper 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 quickly.

The diapers themselves – Although I have never experienced this particular issue, many parents across the web 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 diaper 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.

According to the University of Nebraska Medical Center, 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. As if the health benefits of breastfeeding weren’t good enough, buttered popcorn bowel movements alone are a great reason to keep up the routine for as long as possible! Of course, once solid foods are introduced into the mix, all bets are off and your baby’s poop will probably start smelling like you’d expect it to!

What if the diaper 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 WON’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 really 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 his sweat that smells like maple syrup. It is likely the result of mild dehydration overnight, especially if your baby doesn’t wake for feedings overnight, 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 can 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 sweet smelling breath, drowsiness, intense thirst, heavy breathing, heavy appetite, vision changes, weight loss, and others. If your baby has any of these other symptoms, see a doctor immediately!

What if the diaper smells like yeast?

A yeasty smell, sometimes described by parents as 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 she is 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 antifungals and 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 diaper 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 she isn’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 diaper 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, she can still have issues from the milk if mommy is drinking it before feedings. The best way to check for this is just to eliminate dairy from both of your diets (if breastfeeding) 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.

With all of these different smells, what is NORMAL baby poop?

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.

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!