Final answer:
Baby poop starts to smell when the newborn transitions from meconium, a sterile and odorless substance, to breast milk or formula-fed stools. The change occurs a few days after birth as the baby's diet alters and gut bacteria begin colonizing, leading to smellier stools.
Step-by-step explanation:
The smell associated with baby poop begins to manifest when the baby's diet changes from meconium, which is sterile and odorless, to breast milk or formula. Meconium is the newborn's first stools and consists almost entirely of a tarry, greenish black substance composed of ingested amniotic fluid, cellular debris, mucus, and bile.
Initially, meconium is devoid of bacteria because it is produced while the fetus is in a sterile environment and has not yet consumed breast milk or formula. The transition in stool composition occurs once the baby starts feeding; as bacteria introduced through breast milk or formula begin to colonize the infant's gut, the stools start to have a more noticeable smell. This change typically occurs few days after birth, as the meconium is cleared and the baby's digestive system starts processing the new food.