Baby Poop Smells Like Fish – 10 Possible Causes Explained

A baby’s feces may emit a fishy smell for various reasons, including:

  • Rotavirus: Often accompanied by fever, diarrhea, nausea, cramps, and poop that smells like rotten eggs.
  • Salmonellosis: Often accompanied by watery/bloody diarrhea, vomiting, fever, headache, and cramps.
  • Giardiasis: Often accompanied by greasy/floating poop, watery diarrhea, loss of appetite, bloating, gas, and abdominal pain.
  • Lactose intolerance: Often accompanied by diarrhea, bloating, gas, and cramps.
  • Cholera: Often accompanied by vomiting, dehydration, stomach ache, and watery stool.
  • E. coli: Often accompanied by watery/blood-streaked diarrhea, poor appetite, fever, and vomiting.
  • Certain medications/supplements: Often accompanied by diarrhea and gastrointestinal discomfort.
  • Malabsorption: Often accompanied by constipation, foul-smelling gas, mucus or blood in stool, and watery diarrhea.
  • Metabolic disorders: Often accompanied by fishy-smelling breath and bodily fluids.
  • Diet: Often accompanied by changes to poop consistency, bloating, and gas.

According to Healthline

“In some cases, extremely foul-smelling stool may be an indication of inadequate absorption of nutrients. But if the infant is growing fine and the stool color and consistency are normal, then this may just be normal.”

Consult your baby’s pediatrician if you are concerned about changes to your baby’s stool, especially if you notice other symptoms of illness as well.

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
  • More drowsy or less alert than usual

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

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 Disease), 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.

Because babies poop a lot, breast milk doesn’t spend long in the gut, 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, their poop will become not only smellier but more solid and brown in appearance like adult stools.

Normal Poop Changes in Babies

Normal poop changes in a baby can be influenced by their age, diet, and overall health. Here are some common poop changes that are considered normal:

  • Transition to Solids: When a baby starts eating solid foods, their poop will undergo noticeable changes. It becomes thicker and more formed and takes on different colors depending on the foods consumed.
  • Color Variations: The color of a baby’s poop can vary based on their diet. For example, breastfed babies often have mustard-yellow stools, while formula-fed babies may have stools that are tan, brown, or greenish-brown. Introducing certain foods can also temporarily change the color of poop.
  • Changes in Texture: As a baby’s diet evolves, their poop will change in texture. Initially, it’s soft and mushy, but as they consume more solid foods, it becomes firmer.
  • Frequency Changes: The frequency of bowel movements can fluctuate. Newborns may have multiple bowel movements a day, but as they grow, it’s normal for this to decrease. Some babies may go several days between bowel movements, especially if they are exclusively breastfed.
  • Occasional Straining: Babies might strain or make grunting noises during a bowel movement. This can be normal and is often due to their developing muscles and coordination.
  • Presence of Undigested Food: It’s common to see small pieces of undigested food in a baby’s stool, especially as they are starting to eat solids. This is typically nothing to worry about.
  • Smell Changes: As a baby’s diet diversifies, the smell of their poop may become stronger or take on different odors. This is a natural part of the digestive process.
  • Temporary Diarrhea or Constipation: Babies may experience bouts of diarrhea or constipation from time to time, often due to changes in diet, illness, or the introduction of new foods. Persistent diarrhea or severe constipation should be discussed with a pediatrician.
  • Teething and Digestive Changes: Some parents notice changes in their baby’s poop when they’re teething. Increased saliva production can lead to looser stools.
  • Reaction to Medications or Dietary Changes: Certain medications or changes in a breastfeeding mother’s diet (if breastfeeding) can impact a baby’s poop. This is usually temporary and not a cause for concern.