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 in the lungs can cause inflammation and infection. Meconium can also block the airways, which can cause lung over expansion. If a lung over expands or inflates too much, it can rupture or collapse. Then air from inside the lung can accumulate in the chest cavity and around the lung.