The first stool a baby has is a thick, green, gooey mess called meconium. (For the record, meconium consists of all the stuff babies swallow during his fetal life: skin cells, hair, and other material floating around in the amniotic fluid.) After passing a handful of these sticky poops, a baby has something we refer to as “transition stools.” These poops are runny, green, and foamy. In fact, if your baby has a transitional stool when his diaper is off, you may see it shoot 12 inches through the air! But rest assured, they’re not dangerous and will fade into memory in a couple of days.
When a person’s stomach fills with food, it triggers a reflex that stimulates peristalsis. (Peristalsis is the name for the rhythmic contraction of muscles that push food from the beginning of the gastrointestinal tract to the end.) This explains why adults and older kids often have a bowel movement after eating. However, this reflex is much more active in babies, which is why they often poop while nursing. But the poop that comes out when your baby eats is waste from the feeding that went in hours ago.