Baby Poop Smells Like Fish – 10 Possible Causes Explained

Baby poop famously doesn’t smell like a bed of roses, but a foul odor definitely isn’t the norm for your little one’s waste either.

Healthy breastfed and formula-fed stools can smell yeasty or slightly pungent — so what is behind fishy-smelling baby poop?

Baby poop with a fish-like odor can be the result of intestinal infections such as salmonellosis, cholera, and E. coli. Lactose intolerance, poor absorption in the gut, and certain medications and supplements can also be the cause of foul-smelling poop in infants.

Most babies recover from these types of infections on their own. Still, it’s important to clean all surfaces thoroughly to prevent contamination of your little one.

Let’s look at each of the possible causes for fishy-smelling baby poop, the differences between breastfed and formula-fed poop odor, and when to worry about your little one’s number twos.

Baby Poop Smells Like Fish – 10 Possible Explanations

Fishy-smelling poop in your baby can often be linked to a bacterial infection, but some medications and food intolerances may also be the culprit.

Here are 10 common reasons behind foul-smelling number twos and other symptoms be aware of:

1. Rotavirus Infection

This common viral infection in infants is caused by contaminated surfaces on dirty toys, changing tables, and high chairs and can also come from contaminated food or water sources.

Rotavirus can usually be prevented with a two-dose vaccine given to babies under 6 months.

Other Symptoms To Look For

  • Fever
  • Diarrhea
  • Nausea
  • Abdominal cramps
  • Poop can also smell of rotten eggs

2. Salmonellosis

Salmonellosis is an intestinal infection caused by ingesting contaminated food (especially raw eggs, undercooked meat, or unpasteurized milk).

Babies normally recover from this infection on their own, but be sure to contact your doctor if unpleasant-smelling poop is accompanied by additional symptoms:

Other Symptoms To Look For

  • Watery or bloody diarrhea
  • Vomiting
  • Fever
  • Headache
  • Stomach cramps

3. Giardiasis

Poor sanitation and hygiene around your baby can risk passing on this parasitic infection.

The microscopic parasite Giardia intestinalis can be passed from person to person via the transferral of fecal matter or from contaminated food and surfaces.

Other Symptoms To Look For

  • Poop is greasy and floats in water
  • Watery diarrhea
  • Loss of appetite
  • Bloating
  • Gas
  • Abdominal pain

4. Lactose Intolerance

Lactose intolerance is caused by a lack of the enzyme lactase in the body that helps to break down the lactose into simpler sugars that can then be absorbed more easily in the gut.

Babies lacking this enzyme have a hard time digesting milk and dairy foods.

This is not be confused with a milk allergy, which presents with respiratory symptoms like wheezing.

Other Symptoms To Look For

  • Diarrhea
  • Bloating
  • Gas
  • Stomach cramps
A cute baby with a surprised look on her face during a diaper change.

5. Cholera

Cholera is an infection in the intestines caused by the bacteria Vibrio cholerae.

This occurs when the body ingests contaminated food or water.

Most babies will experience mild to moderate stomach aches with cholera, but in cases accompanied by the rare symptom of severe dehydration, this can be fatal.

Other Symptoms To Look For

  • Vomiting
  • Severe dehydration (rare)
  • Stomach aches
  • Watery stools

6. E. Coli

An Escherichia coli or E. coli infection can be caused by ingesting food or water (commonly found in contaminated ground beef or unpasteurized cow’s milk).

E. coli can also be passed from the mother’s genital tract to newborns in childbirth.

In most cases, symptoms usually disappear on their own within a week, while some babies may require a course of antibiotics.

Other Symptoms To Look For

  • Watery, sometimes blood-streaked diarrhea
  • Poor appetite
  • Fever
  • Vomiting

7. Certain Medications/Supplements

Speaking of antibiotics, some medications can cause foul-smelling poop in your baby as can certain supplements like over-the-counter multivitamins, according to Healthline.

This is possibly caused by an allergic reaction to an ingredient or taking more than the daily allowance of a certain vitamin.

Other Symptoms To Look For

  • Diarrhea
  • Gastrointestinal discomfort

Also, check here for the symptoms linked to a specific vitamin or mineral overdose.

A mother holding her nose during a smelly diaper change.

8. Malabsorption

This occurs when your baby’s body struggles to absorb an adequate amount of nutrients from food.

It is usually due to an infection, forms of IBD (Inflammatory Bowel Diseases), food allergies, or celiac disease, hindering the intestine’s ability to absorb well.

Other Symptoms To Look For

  • Constipation
  • Foul-smelling flatulence
  • Mucous or blood in stool
  • Watery diarrhea

9. Metabolic Disorders

In some cases, a fishy poop smell can be caused by a rare, often inherited metabolic condition known as trimethylaminuria (TMAU) or “fish odor syndrome.”

This disorder can be present from birth and essentially means that the body can’t process the bad-smelling chemical trimethylaminuria, causing levels of this chemical to build up until your bodily waste smells fishy.

Other Symptoms To Look For

  • Fishy-smelling breath, sweat, urine, or vaginal fluids

10. Your Baby’s Diet

Once your baby is introduced to solids, the odor of their poop will change, but foul-smelling poop can indicate an allergy to something they have eaten.

Studies have also shown that formula resulted in poop containing more sulfur gases compared with the poop of breastfed babies.

Other Symptoms To Look For

  • Changes to poop consistency
  • Bloating
  • Gas

Breastfed Baby Poop Smell

The poop of a breastfed baby shouldn’t have too much of an odor at all.

This is because breast milk particles are tiny enough that they are readily absorbed in the gut, and because babies poop a lot, breast milk doesn’t spend long in the gut either, meaning less opportunity for bad-smelling bacteria to accumulate.

If there is a smell to breastfed poop, it can usually be described as sweet, yeasty, or cheesy.

Formula Fed Baby Poop Smell

Formula-fed baby poop will have a more noticeable smell since the particles in formula are much bigger than those in breast milk, meaning it takes longer to process in the gut, building up more bacteria.

According to Breastfeeding Support, once your little one is introduced to formula or solids, this is when their poop will become not only smellier but more solid and brown in appearance like adult stools.

When To Worry

It’s important to contact your doctor promptly if your baby has a sudden change in their poop’s smell, frequency, or consistency (from solid to watery) and this change is accompanied by symptoms such as:

  • Fever
  • Loss of appetite
  • Irritability
  • Vomiting
  • Change in level of consciousness, i.e., more drowsy or less alert than usual

Any of the above may indicate an underlying illness or condition that requires further attention.

Final Thoughts

Fishy-smelling baby poop can be caused by viral and bacterial infections as a result of contaminated food and surfaces, so always take care when preparing food and maintaining a hygienic environment for your little one.

In some cases, medication and your baby’s diet can also be the cause of foul-smelling poop.

If a fishy poop odor is ever alongside any of the additional symptoms mentioned above, please be sure to see your doctor straight away as some of these signs can indicate a more serious reaction to a common infection or an underlying medical condition.