Background
Meconium aspiration syndrome (MAS) is the aspiration of stained amniotic fluid, which can occur before, during, or immediately after birth. Meconium is the first intestinal discharge from newborns, a viscous, dark-green substance composed of intestinal epithelial cells, lanugo, mucus, and intestinal secretions (eg, bile. Water is the major liquid constituent, comprising 85-95% of meconium; the remaining 5-15% of ingredients consists of solid constituents, primarily intestinal secretions, mucosal cells, and solid elements of swallowed amniotic fluid, such as proteins and lipids.
Meconium is sterile and does not contain bacteria, which is the primary factor that differentiates it from stool. Intrauterine distress can cause passage of meconium into the amniotic fluid. Factors that promote the passage in utero include placental insufficiency, maternal hypertension, preeclampsia, oligohydramnios, infection, acidosis, and maternal drug abuse, especially use of tobacco and cocaine.
As noted above, meconium-stained amniotic fluid may be aspirated before or during labor and delivery. Because meconium is rarely found in the amniotic fluid prior to 34 weeks’ gestation, meconium aspiration primarily affects infants born at term and postterm.