Meconium is the first stool of an infant. Sometimes this stool is passed into the amniotic fluid that surrounds the baby in pregnancy.

Meconium aspiration is when the meconium is inhaled into the baby’s lungs. Meconium aspiration syndrome (MAS) is a set of symptoms that may result from meconium inhalation.

Not all infants who inhale meconium will develop these symptoms. It is not known why some infants develop MAS and others do not.