Why does my baby's poop smell like rotten eggs or sulfur?

The rotten egg smell is hydrogen sulfide gas, produced when sulfur-reducing bacteria in the gut break down sulfur-containing compounds. The most common triggers: a stomach virus, eating sulfur-rich foods (eggs, cruciferous vegetables, meat, legumes), a formula change, or starting solid foods. It almost always resolves on its own within a few days once the cause clears.

Is rotten egg-smelling poop a sign of a stomach bug?

It can be. Viral gastroenteritis — caused by rotavirus, norovirus, or similar bugs — is one of the most common reasons for a sudden sulfur smell. It usually comes with looser stool, more frequent diapers, and possibly a mild fever. Most stomach bugs resolve in 3-5 days. If your baby has 6 or more watery stools in a day, a fever above 100.4°F, or any signs of dehydration (no wet diapers for 6+ hours, dry mouth, no tears), call your pediatrician.

Can what I eat cause my breastfed baby's poop to smell like sulfur?

Yes. Cruciferous vegetables — broccoli, cauliflower, Brussels sprouts, cabbage, kale — contain sulfur compounds called glucosinolates. When you eat them, those compounds pass into breast milk and can affect your baby's gut bacteria within 24-48 hours. Eggs are also high-sulfur. If you ate a lot of these foods recently and now have a sulfurous-smelling diaper on your hands, that's almost certainly why. It clears once those foods cycle out of your system.

How long does sulfur-smelling poop last in babies?

Depends on the cause. If it's a stomach bug, expect 3-5 days. If it's dietary (something you ate while breastfeeding, or a new solid food baby just tried), it usually resolves within 1-3 days once that food is out of the system. If you switched formulas recently and the smell started around then, give it a week — if it persists, ask your pediatrician about trying a different formula.

When should I call the doctor about bad-smelling baby poop?

Call if the smell comes with: fever above 100.4°F (38°C) in any baby, or any fever at all in a baby under 3 months; 6 or more watery stools in a day; signs of dehydration (no wet diapers in 6+ hours, dry mouth, sunken fontanelle, no tears when crying); blood in the stool; or baby who seems lethargic, won't eat, or looks genuinely sick. Smell alone, without these red flags, is usually not urgent.

Baby Poop Smells Like Rotten Eggs: What It Means

You opened the diaper and got hit with something that smelled unmistakably like rotten eggs. Now you're wondering if something is wrong.

Almost certainly not. That sulfur smell has a specific cause — hydrogen sulfide gas — and the reasons it shows up are mostly ordinary. Here's what's actually going on.

What causes the rotten egg smell

The smell is hydrogen sulfide, a gas produced when sulfur-reducing bacteria in the gut break down sulfur-containing compounds. Every gut has these bacteria. What changes is how much sulfur they have to work with — and that's where your specific cause comes in.

1. Stomach bug (most common)

Viral gastroenteritis — rotavirus, norovirus, or similar — is the most frequent culprit for a sudden, out-of-nowhere sulfur smell. The virus disrupts the normal gut environment, which shifts bacterial populations and temporarily ramps up hydrogen sulfide production.

Other signs this is what you're dealing with: looser or more frequent stool than usual, mild fever (100-101°F), baby seeming off or fussy. Most resolve in 3-5 days without treatment beyond keeping baby hydrated.

2. Sulfur-rich foods in the diet

Certain foods are naturally high in sulfur compounds, and when they get digested, that sulfur ends up in the gut — where bacteria turn it into hydrogen sulfide.

The main offenders:

Eggs — very high sulfur protein
Cruciferous vegetables — broccoli, cauliflower, Brussels sprouts, cabbage, kale. These contain glucosinolates, sulfur-containing compounds that produce gas during digestion.
Meat and legumes — high protein, higher sulfur

If you're breastfeeding: what you eat shows up in your breast milk, usually within 4-6 hours, and can affect baby's poop 24-48 hours after your meal. If you had a big plate of broccoli or eggs yesterday, that timing lines up.

If baby just started a new solid food from this list, that's almost certainly why the smell changed.

3. Formula change

Different formulas use different protein sources, and some are significantly higher in sulfur. If the smell started within a few days of switching formulas, the new formula may just produce more hydrogen sulfide in your baby's gut. Give it a week to settle. If it doesn't, or if baby seems uncomfortable, ask your pediatrician about a different formula.

4. Starting solid foods

This is one of the most consistent triggers. The shift from an all-milk diet to solid food changes the gut microbiome significantly. When baby starts eating eggs, meat, legumes, or cruciferous vegetables for the first time, the bacteria in the gut encounter far more sulfur than they're used to, and the smell reflects that. It usually stabilizes as the gut adapts.

5. Gut microbiome disruption

After a course of antibiotics, or after an illness, the balance of bacteria in the gut shifts. This can temporarily alter how much sulfur is being metabolized. It typically normalizes on its own over 1-2 weeks, though probiotic foods (if baby is eating solids) can help the microbiome recover.

How long does it last?

Stomach bug: 3-5 days, sometimes up to a week. Dietary cause: 1-3 days once the food clears your or baby's system. Formula-related: a week or so to adjust, or until you switch. Microbiome disruption after antibiotics: 1-2 weeks.

If it's been more than 2 weeks and nothing has changed, that's worth a call to your pediatrician.

When to call the doctor

The smell by itself isn't a reason to call. These are:

Fever above 100.4°F (38°C) — or any fever in a baby under 3 months
Six or more watery stools in a day
No wet diapers in 6+ hours, dry mouth, sunken soft spot, no tears when crying (dehydration)
Blood in the stool
Baby seems lethargic, is refusing to eat, or looks genuinely unwell

If you're seeing the sulfur smell with just looser stool and a mildly unhappy baby, you're almost certainly in stomach-bug territory. Keep baby hydrated and wait it out.

The broader smell picture

Sulfur is one of many ways baby poop smell can change. For a full rundown of what different smells mean — sour, acidic, metallic, foul — see our guide to baby poop smell.

Track it so you have data

When your pediatrician asks how long the smell has been off, or how many loose stools a day baby is having, guessing from memory rarely ends well. PipPoopie lets you log smell, consistency, and frequency for every diaper, so you have an actual timeline instead of an approximation when you need it.