Baby Poop Smell Like Fish (Here’s Why)
If you’ve ever changed a diaper and thought, “Why does this smell like something I’d find at a seafood counter?”- you’re not alone.
Baby poop can come with some truly confusing smells, and a fishy one tends to stop parents in their tracks.
The good news is this: most of the time, a fish-like smell has a pretty normal explanation.
Babies have developing digestive systems that are still figuring things out, and that process can get… complicated. Smell is part of the deal, even when everything else is perfectly fine.
In this post, we’ll explain why baby poop smells like fish, what’s normal, what’s not, and when you should loop in your pediatrician.
#1. Normal Digestive Bacteria
A baby’s gut is basically a work in progress.
At birth, their digestive system doesn’t have much bacteria at all. Over time, it slowly builds a balance of “good” bacteria that help break down milk and nutrients.
During that adjustment period, some odd smells can show up.
When milk gets digested, certain bacteria produce sulfur-like compounds. Those compounds can mix together in a way that smells fishy, metallic, or just plain weird.
So it just means the gut bacteria are still learning how to play nice together.
This is especially common in newborns and younger infants. Their digestion changes fast in the first few months, so the smell of poop can change just as quickly. One week it’s barely noticeable, the next week it smells like fish.
Totally normal.
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#2. Formula Can Change The Smell
If your baby drinks formula, smell changes are even more common.
Formula tends to produce stronger-smelling poop than breast milk, and some formulas are more noticeable than others.
Different formulas use different protein sources and fat blends. Some are cow’s milk-based, some are partially broken down, and some are designed for sensitive stomachs. All of those digest differently, and digestion directly affects smell.
A fishy odor can pop up when a formula doesn’t break down as smoothly in your baby’s gut.
This doesn’t automatically mean the formula is bad or hurting your baby. Many babies thrive on formulas that still create funky diapers.
If the smell changes suddenly after switching formula, that’s usually your explanation. In many cases, things settle down after a few days once your baby adjusts.
#3. Breast Milk Diet Effects
Breastfed babies can have fishy-smelling poop too, and sometimes the reason is sitting right on your dinner plate.
What a nursing parent eats can influence breast milk. Foods like fish, seafood, garlic, onions, and even omega-3 supplements can slightly change the smell of a baby’s poop.
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Vitamins and certain medications can do it as well.
This doesn’t mean you need to panic or stop eating nutritious foods. It’s more of an interesting side effect than a problem. If your baby seems comfortable, feeds well, and has normal diapers otherwise, the smell alone usually isn’t a concern.
Most of the time, the smell fades as the food passes through your system and your baby’s system catches up.
#4. Mild Diarrhea Or Faster Digestion
Sometimes baby poop smells fishy simply because it moved through the digestive system too fast. When digestion speeds up, food doesn’t fully break down.
That can create sharper, stronger odors.
This can happen during:
Teething
Minor stomach bugs
Small digestive upsets
Growth spurts
In these moments, you might notice poop that’s looser, more frequent, or slightly different in color. As long as your baby stays hydrated, feeds normally, and acts like themselves, this type of smell change usually passes on its own.
#5. Infections Or Imbalance (Less Common)
Now let’s go over the less common stuff, without jumping straight to panic mode.
A strong fishy smell paired with other symptoms can sometimes point to a gut infection or bacterial imbalance. This doesn’t happen often, but it’s something to keep in mind if the smell is intense and doesn’t go away.
Infections can irritate the digestive tract and change how stool smells, looks, and feels.
In those cases, smell isn’t the only sign. There are usually other clues that something’s off.
This is where context matters. One weird diaper on its own isn’t a red flag. A pattern plus other symptoms is when it’s worth paying closer attention.
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#6. Rare Metabolic Conditions
There are a few super rare metabolic conditions that can cause a strong fish-like odor in sweat, urine, and stool.
One example is trimethylaminuria, often called “fish odor syndrome.”
This is EXTREMELY uncommon, and it usually shows up consistently, not just in diapers. The smell tends to be strong, persistent, and noticeable beyond poop alone.
Doctors only consider this after ruling out far more common causes.
If you’re reading this and thinking, “This sounds scary,” take a breath. For the vast majority of babies, this is not the issue.
When It’s Usually Nothing To Worry About
Most of the time, fishy-smelling baby poop is just part of normal baby life. You can usually relax if things look like this:
Your baby is eating normally and seems content
No fever, vomiting, or unusual fussiness
Poop color stays in a normal range
The smell comes and goes instead of sticking around
Babies change constantly, and their diapers change right along with them.
Smell alone, without other symptoms, is rarely a problem.
When To Call The Pediatrician
Trust your instincts here. If something feels off, it’s always okay to check in. You should call the pediatrician if you notice:
The fishy smell lasts several days without improvement
Stool looks gray, white, or very pale
There’s blood or lots of mucus in the diaper
Your baby seems uncomfortable, lethargic, or feeds poorly
Doctors hear questions like this all the time. You won’t sound silly or dramatic for asking.
Bottom Line
Fishy-smelling baby poop is usually linked to normal digestion, formula differences, diet changes, or temporary stomach issues.
It can be surprising, unpleasant, and confusing, but most of the time it’s harmless.
If your baby is acting like their usual self, eating well, and growing normally, the smell alone isn’t something to stress over. Keep an eye on patterns, trust your gut, and reach out to your pediatrician if anything else shows up alongside the smell.
Parenthood comes with a lot of unexpected moments. Weird diapers are definitely one of them.