Meconium is the passage of the baby's stool. It is a bowel movement that baby makes which doesn't usually happen before delivery unless the baby is stressed during delivery and usually is in the amniotic fluid. Meconium aspiration syndrome (MAS) happens when a newborn has trouble breathing because meconium got into the lungs, causing a reaction in the lungs. Meconium can make it harder to breathe because it can: clog the airways irritate the airways and injure lung tissue block surfactant, a fatty substance that helps open the lungs after birth