During pregnancy, the intestinal tract of a fetus is lined with a dark green fecal material called meconium. While in the uterus, a number of factors can cause increased movement of the intestines and relaxation of the anal sphincter. This results in passage of meconium into the amniotic fluid. When this happens, the amniotic fluid and meconium mix to form a thick, green-stained fluid.

MAS can develop if the baby gasps or inhales during labor and delivery, causing the meconium mixture to be aspirated into the lungs. Fetal distress and post-term delivery are the most common reasons for meconium aspiration to develop, but it is also associated with small-for-gestational-age infants, umbilical cord complications, chronic medical conditions, and poor intrauterine growth.

Meconium aspiration can cause a partial or complete blockage of the baby’s airways. The meconium becomes trapped in the airways when the baby exhales. The involved lung may become over expanded. Over-expansion of a portion of the lung can eventually result in rupture and then collapse of the lung. Meconium also causes irritation to the baby’s airways and lungs.

Treatment for MAS may include suctioning the airway, antibiotics, use of a ventilator or artificial breathing machine, and chest physiotherapy.