Breastfed Baby Breath Smells Like Sour Milk: Understanding The Mysteries

Written by Daisy
Last updated:
Reviewed by Margaret

The article has been reviewed by medical expert Margaret Dogwood, BSN, RN

Ever wondered why breastfed baby breath smells like sour milk? As it turns out, most of the time, this emanation is caused by gastroesophageal reflux, commonly referred to as GER ­– or infant acid reflux, for short. I know how new parents can find this rather dangerous, but it’s usually not something to be afraid of. However, other possible reasons why this is so are there, and that’s what we’ll talk about today!

So, in this article, we’ll be diving deeper into what’s causing this funny smell to come out of our infants’ mouths. We’ll also unravel when you can shrug it off when it’s time to give your pediatrician a ring, and many more!

The Natural Aroma of Breastfed Baby Breath

It could either be underwhelming or overwhelming to smell the unusual scent of your baby’s breath, but hey, think about what they’re consuming.

For up to six months, your baby will ONLY be consuming either breast or formula milk, making the natural scent of their breath subtly sour. Baby breath that has been exclusively breastfed has a particular natural odor that is sometimes referred to as sweet or milky. It is thought to be impacted by a number of variables, including the makeup of breast milk, the infant’s oral microbiome, and the metabolism of particular breast milk constituents.

This isn’t actually a cause for concern, unless, of course, you sense something different. Let’s cut to the chase – what’s actually making their breaths smell pungent?

Why Breastfed Baby Breath Smells Like Sour Milk? 4 Factors Contributing to a Sour Milk Odor

As much as we want to think that our babies’ breaths are simple – they’re not. Even our own breaths can’t be tied up to a single circumstance of why they’re so.

Luckily, the smell of baby breath can easily be diagnosed because of the fact that they’re only consuming milk. Here are the four different factors that contribute to the sour milk scent from your babies’ breaths!

Residual Milk

Probably the most common reason of all – and why you shouldn’t be frantic about it is because of residual milk.

Milk that is left behind after nursing may be found in the infant’s mouth – don’t expect them to be able to flush it out of their systems, yet! If the milk residue is not thoroughly cleaned, it may begin to degrade and produce this subtly unpleasant odor.

After feedings, ensuring good oral hygiene by gently cleaning the baby’s gums and tongue will help lessen the presence of leftover milk and lessen the smell of sour milk.

The good news is – there’s nothing to be afraid of!


Apart from residual milk, sugar can also be a factor.

A baby’s mouth could be filled with bacteria from the natural sugars found in their mommy’s breast milk. These bacteria’s metabolism of the carbohydrates can result in acidic metabolites that help explain why the milk has a sour scent.

Experts from the ADA or the American Dental Association, say that milk can actually host bacterial growth in the mouth, which can lead to a sour and pungent scent.

             Additionally, our lips serve as a natural incubator for the growth of these bacteria. When you eat, bacteria in your mouth consume the food that is still there and produce an odorous waste product.”

So, what do we parents need to do – clean our babies’ mouths regularly, but carefully!

Dry Mouth

The taste of sour milk may be made worse by a dry tongue. I mean, think about your own condition – don’t dry mouths smell bad, especially if they’ve been dry for a while?

Saliva flow decreases when the baby’s mouth is dry, which helps to keep the mouth clean and maintain a balanced population of germs. Regular breastfeeding and adequate drinking can both prevent dry mouth and encourage salivation.

WebMD confirms that dry mouth can actually make matters worse.

             Halitosis develops if there isn’t saliva to wash the germs out of their mouths. Your kid may have dry mouth if they breathe through their mouth.”

Gastroesophageal Reflux

Last but most definitely not least would be the infamous GER or the gastroesophageal reflux. This probably is the least common out of all four, and also the rarest that could happen.

In rare instances, gastric reflux might be linked to the sour milk smell on a breastfed baby’s breath. When milk hasn’t completely digested or stomach acid runs up into the esophagus, GER ensues. The sour scent may be caused by the milk that was regurgitated.

NOTE: It will be best to consult with a healthcare provider for accurate assessment and advice if you believe your infant may have GER.

These can’t be everything there is, right? Well, sadly, that’s true. There are a couple of possible medical causes on why you’re smelling something unusual from your babies’ breaths.

Possible Medical Causes of Infant Bad Breath

They’re people, and, as people, there are a couple of medically-inclined conditions that could trigger the scent you’re noticing.

Pediatric Sinusitis

The first and probably the most common medical condition that causes babies’ breaths to have this unpleasant smell would be sinusitis. 

When infants’ sinuses swell, mucus and germs can build up. This can cause a bad odor (that resembles sourness and pungence) to be exhaled via the mouth.

Chronic Kidney Disease

Babies with chronic renal disease may experience difficulty getting rid of waste materials from their bodies through their kidneys. Halitosis, or foul breath, can develop as a result of the buildup and discharge of these waste products through the breath.

Enlarged Tonsils

When newborns’ tonsils grow larger, food particles and bacteria might become caught in the fissures. The bacterial growth may result in an unpleasant odor, which aggravates foul breath.

Udayan K. Shah, MD, Pediatric Otolaryngology, also says that this condition can worsen and lead to unwanted activity like coughing and nosebleeds.

             Larger tonsils can cause nosebleeds, bad breath, and coughing, too.”

Possible Diabetes

Lastly, diabetes. uncontrolled diabetes can result in the formation of ketones. Ketones are these noxious-smelling compounds that can resonate and spread going to the mouth. Ketones in an infant’s breath might result in bad breath if their diabetes is misdiagnosed or poorly managed.

Teething and Baby’s Sour Breath

Have you ever heard of teething? Well, in the simplest terms possible, teething is when the gums of your babies move for the breakthrough of teeth – it’s when they grow their first milk teeth! Unfortunately, it’s not just about their teeth.

First of all, babies with teething frequently produce more saliva. This is the body’s natural way of protecting and soothing the inflamed gums of your cute little ones.

The mouth can instantly become clogged with extra saliva, combined with other things like milk or food particles to produce a more overt odor.

Additionally, due to its contact with microorganisms in the mouth, the increased saliva might contribute to a sour milk odor. In the oral cavity, bacteria normally exist, and when they interact with extra saliva, they can decompose and ferment leftover milk or food.

The volatile sulfur molecules that might be produced throughout this fermentation process are what give off the sour scent.

What they’re experiencing is already painful, so try and support them! It is crucial to remember that while changes in breath odor are common during teething, they are typically temporary and shouldn’t raise any red flags unless they are coupled with other symptoms like fever, extreme fussiness, or poor eating.

Oral Hygiene and Baby Breath

Other than that sour milk smell, there are other scents and smells that you could notice from your babies’ breaths!

Metal Breath

When newborns exhibit mental breath, it may be the result of whatever they’ve put in their mouths. Frequently, our babies put objects in their mouths to explore the environment, and if they come into touch with metal toys and other objects, their breath can reflect this smell.

Sweet Breath

Babies’ sweet breath can happen for a number of different reasons. Consuming sugary foods or beverages, such as fruit juices or purees, is one typical reason. A baby’s medicine may also be a factor in the development of sweet breath if it contains sugars or other sweeteners.

For newborns, however, it can be a result of the health condition known as diabetic ketoacidosis (DKA). DKA happens when the body doesn’t produce enough insulin, which raises blood sugar levels. In reaction, the body uses the ketones that are created when fats are broken down for energy, which might leave you with a sweet or fruity breath odor.

Cheese Breath

A baby’s food may be the cause of their cheese breath. A baby may smell like cheese whenever they ingest dairy products or breast milk with a high fat content. This is due to the possibility that the mouth’s breakdown of specific lipids and proteins will result in the production of volatile sulfur compounds that cause the odor.

Vomit Breath

Infants who have recently vomited or spited up may have vomiting breath. A sour or acidic odor may remain in the mouth after stomach contents have returned there. If the infant has reflux problems or has not been properly burped, this may be more obvious.

Reflux or gastroesophageal reflux disease (GERD) are two conditions that can also lead to vomit breath in neonates. In these circumstances, stomach acid refluxes into the esophagus and produces a foul stench that resembles vomit.

Fish Breath

If a baby consumes fish or other seafood, their diet may be the cause of their fish breath. Fishy breath can result from eating certain kinds of fish, particularly fatty fish or seafood with a strong fragrance.

Additionally, this can also be linked to a metabolic condition called trimethylaminuria, sometimes known as “fish odor syndrome,” which is brought on by an inability to break down a substance called trimethylamine, causing the emission of volatile chemicals with a fishy stench.

Maintaining oral hygiene is the way to go! After feeding, it is important to clean your baby’s mouth and tongue to maintain good oral hygiene.

Start early and begin to clean your babies’ mouths when they’re teething. Yes! Clean it before those pesky milk teeth even show up! Use a damp soft cloth or an infant toothbrush (better) to carefully and successfully clean their teeth and gums.

When they’re at about six to eight months old, transition to a toothbrush. This allows their gums to be desensitized and accept the soft bristles, which will go on to be as tough as what we adults use daily!

NOTE: Just be sure to clean their teeth and gums gently as rapturing it can cause an even bigger problem!

When To Seek Professional Medical Advice

Now, we’re more understanding of what’s happening when we notice the sour and pungent smell coming out of our babies’ mouths. However, when should we be concerned about it? When do we seek professional medical advice?

A baby’s breath smell raises many warning signs that demand medical attention:

–          Persistent Bad Odor: It is best to seek medical assistance if the baby’s breath continuously has a foul or overpowering stench.

–          Odd Breath Odors: It’s crucial to visit a healthcare practitioner for additional investigation if the baby’s breath exhibits an odd odor that is not generally connected to regular eating or oral hygiene – or if it isn’t part of what’s discussed above!

–          Associated Symptoms: It is advised to seek medical care right once if the infant displays further alarming signs including fever, extreme fussiness, slow weight gain, or changes in feeding habits.


In case you’re still left with a couple of questions, here are some of the most commonly asked questions about our babies’ breaths smelling like sour milk!

Why Does My Newborn’s Breath Smell Bad?

There are a number of causes for why your newborn’s breath can smell foul. Remaining milk in their mouth after feeding is the most frequent reason. This milk residue might deteriorate and provide a mildly unpleasant odor if it is not adequately cleaned. The natural sugars in breast milk can also be broken down by the bacteria in your baby’s mouth, producing acidic byproducts and a sour milk smell.

Breastfed Baby Breath Smells Like Poop

Baby breath from breastfed babies often has a somewhat distinct fragrance, but if it smells like excrement, there may be a few things to think about, including digestive issues and improper oral hygiene.

Final Verdict

Do you now understand why breastfed baby breath smells like sour milk? You see, it’s not something to actually be afraid of. 9 times out of 10, it’s just because of what they consume or at the least, acid reflux – which is also not that alarming!

Should your baby’s breath exhibit more than what’s expected, that’s when you should extend the request for professional medical assistance. 

Photo of author
Daisy Martinez, a Certified Financial Planner and mother of two, blends finance expertise from Ohio State and Anderson School of Management with hands-on parenting insights. Founder of "" and "", she's passionate about guiding parents in financial and parenting realms.

Get a freebie every week!

Join 40,000+ parents and teachers , and you'll get a freebie in your inbox at least once a week, along with tips, information, and other resources!

Subscribe to the Mamallove newsletter: