During the first few days after delivery, infants don't produce smelly bowel movements. The odorless, greenish-black material you first find in your newborn's diaper is called meconium, and it started forming in her bowels at 16 weeks of gestation. Once babies process this waste, regular bowel movements start to occur. The stink factor of infant stool varies widely from child to child and depends on multiple factors, including your baby's diet, medications and health.
Smelly bowel movements in breastfed infants are rare, unless the child is sick or suffers from a digestive problem. As a rule, expect the stool to have a pale or bright yellow color and mild scent. Most breastfeeding mothers also notice yellow, seed-like structures scattered throughout the feces, which is usually slightly soft and runny. The mild, somewhat sour smell of a breastfed baby's stool may become more intense or nose-wrinkling if a baby is taking medication, such as antibiotics, or is sensitive to a particular food that his mother consumes, such as dairy products or gas-producing vegetables such as broccoli.
Babies fed formula often produce stool that has a strong or unpleasant odor. In general, the normal feces from a typical formula-fed baby is firm, brown and quite smelly. Formula-fed babies can't absorb man-made formula as well as breast milk, so their bowel movements contain more waste, which produces a more potent odor, according to Catharine Parker-Littler, registered midwife and author of "Ask a Midwife."
A small study published in the May 2001 issue of "The Journal of Pediatric Gastroenterology and Nutrition" sheds further light on the difference in odor between stool from breastfed and formula-fed infants. Study authors, led by Dr. Ekhard Ziegler, University of Iowa professor of pediatrics, collected and examined stool samples from babies fed breast milk, soy-based formula and cow milk-based formula. Stool from breast milk-fed babies produced high amounts of odorless hydrogen gas and low levels of foul-smelling sulfur gases. Stool from the formula-fed babies primarily produced a mix of odor-causing sulfur gases, including methyl mercaptan and hydrogen sulfide, a gas distinguished by its rotten-egg smell.
Regardless of your baby's food source, repeated instances of very loose, smelly bowel movements typically indicate diarrhea. Depending on the cause, other symptoms that often occur with diarrhea include irritability and fussiness, fever, abdominal cramps and vomiting. Diarrhea in infants could become quite serious and may lead to severe dehydration and death if left untreated. Prevent these serious complications by contacting your baby's pediatrician as soon as you notice watery, foul-smelling stool, especially in an infant under the age of 3 months or one who shows signs of dehydration.