A rotten egg smell in your baby’s diaper usually comes from sulfur gases produced by bacteria in the gut. It’s one of the most common stool changes parents notice, and in most cases it traces back to something your baby ate or drank rather than a medical problem. That said, certain patterns are worth paying attention to.
What Creates the Sulfur Smell
The rotten egg odor is hydrogen sulfide, a gas that gut bacteria release when they break down sulfur-containing proteins and amino acids. Every human gut produces some hydrogen sulfide, but the amount varies depending on what’s being digested and which bacteria are doing the work. In babies, the balance of gut bacteria is still developing, which means stool odor can shift noticeably from week to week as new bacterial colonies establish themselves.
Research on infant fecal gas production has found that the type and quantity of sulfur gases vary significantly based on diet. Soy-formula-fed infants, for example, produced notably high levels of hydrogen sulfide compared to other feeding groups. This makes sense: soy protein is rich in sulfur-containing amino acids, which give gut bacteria more raw material to convert into sulfur gas.
Formula, Breast Milk, and Stool Odor
Breastfed babies generally have milder-smelling stool. For many exclusively breastfed infants, the poop barely has an odor at all. Formula-fed babies tend to have a slightly stronger smell even under normal circumstances, because formula proteins are digested differently and support a somewhat different mix of gut bacteria.
If you recently switched formulas, especially to a soy-based or high-protein variety, that alone could explain the new sulfur smell. The same applies if you’re breastfeeding and you’ve changed your own diet significantly, since compounds from your food do pass into breast milk.
Solid Foods That Trigger the Smell
Once your baby starts solids, stool odor changes dramatically. Certain foods are especially high in sulfur compounds and are common culprits behind the rotten egg smell:
- Cruciferous vegetables: broccoli, cauliflower, Brussels sprouts, and cabbage
- Eggs: one of the most sulfur-dense foods
- Beans and legumes
- Dairy products: milk, cheese, yogurt
- Garlic, onions, and asparagus
- High-protein foods: fish and poultry
If the smell appeared right around the time you introduced one of these foods, that’s likely your answer. The odor typically settles as your baby’s gut adjusts to the new food over several days. You don’t necessarily need to stop offering these foods. They’re nutritious. But spacing out new introductions helps you identify which ones your baby’s gut reacts to most strongly.
Lactose Intolerance and Malabsorption
When a baby can’t fully break down lactose, the sugar in milk and dairy, gut bacteria ferment it instead. That fermentation produces gas and can make stools smell significantly worse. Lactose intolerance in infants typically shows up as a cluster of symptoms: foul-smelling or unusually frequent stools, bloating, gas, and fussiness during or after feeding. The smell and frequency often increase with the amount of dairy consumed.
Protein malabsorption can produce a similar pattern. When proteins aren’t properly broken down and absorbed in the small intestine, they reach the colon intact, where bacteria ferment them and release sulfur gases. The hallmarks are greasy, voluminous, foul-smelling stools that may appear to float. In infants, conditions like cystic fibrosis can impair the enzymes needed for protein digestion, though this is uncommon and typically identified early through newborn screening.
If the smell persists regardless of dietary changes, or if your baby seems to have pain, excessive gas, or poor weight gain alongside the odor, a food intolerance or absorption issue is worth exploring with your pediatrician.
Infections That Change Stool Odor
A sudden, dramatic shift to foul-smelling stool, especially paired with diarrhea, can signal a gut infection. Giardia, a waterborne parasite, is one of the classic causes of sulfur-smelling stool in children. It produces explosive, watery, greasy stools with a distinctly foul odor, along with bloating, nausea, and fatigue. Babies can pick up giardia from contaminated water or surfaces at daycare.
Bacterial infections from organisms like salmonella or rotavirus can also produce intensely foul stools. The key difference between a dietary cause and an infection is the speed and severity of the change. A food-related sulfur smell develops gradually and isn’t accompanied by illness. An infection usually brings sudden onset diarrhea, and your baby will seem visibly unwell.
Signs That Need Medical Attention
A sulfur smell on its own, with otherwise normal stool and a happy baby, is rarely a concern. But contact your pediatrician if the smell comes alongside any of these:
- Fever
- Vomiting
- Severe or watery diarrhea
- Blood or mucus in the stool
- Poor weight gain or weight loss
Watch for dehydration signs as well, particularly if diarrhea is involved. Fewer than six wet diapers in 24 hours, a sunken soft spot on the head, unusual sleepiness, increased irritability, or few tears when crying all point to dehydration that needs prompt attention. For babies 3 months or younger, any episode of diarrhea warrants a call to your pediatrician regardless of other symptoms.
What You Can Do at Home
Start by tracking what your baby has eaten in the past 24 to 48 hours. If you recently introduced a high-sulfur food, try pulling it back for a few days and see if the smell resolves. For formula-fed babies, note whether the odor started with a formula change.
Keep in mind that some degree of stool odor change is completely normal as your baby’s gut matures. The bacterial ecosystem in an infant’s intestines is in constant flux during the first year, and each new food, each minor illness, and even teething can temporarily alter what comes out the other end. A few notably smelly diapers in an otherwise healthy, growing baby are almost always just part of the process.