Meconium aspiration occurs when a baby breathes in amniotic fluid containing meconium (the baby's first stools). Before or during labor, the fetus sometimes passes the meconium stool into the amniotic fluid. It is not clearly understood why this happens. It may be a natural event, but it is also thought to be related to fetal distress in some babies. When the thick meconium mixes into the amniotic fluid, it is swallowed and breathed into the airway of the fetus. As the baby takes the first breaths at delivery, meconium particles enter the airway and can be aspirated (inhaled) deep into the lungs.