CLINICAL ASPECTS OF EARLY EMBRYONIC DEVELOPMENT Meconium Aspiration Syndrome (MAS)

Although the pathway for fetal defecation exists, this does not usually occur. In most cases the first defecation occurs postnatally; the bile-stained fecal matter is called meconium. In 10 15% of all deliveries, the neonate's amniotic fluid is meconium-stained, indicating some amount of in utero defecation. Even though about one-third of such neonates will have some meconium in their lungs, only about 5% of children born with meconium-stained amniotic fluid will actually have a condition known as Meconium Aspiration Syndrome. This potentially serious disease is characterized by respiratory distress and pneumonia. It used be thought that the toxic affect of meconium on pulmonary tissue caused MAS. However, the most recent article I read suggests that the neonatal respiratory distress is primarily caused by pulmonary arteriolar vasoconstriction that is a byproduct of a pre-existing fetal asphyxia unrelated to meconium in the amniotic fluid. It does seem that a lung with pulmonary arteriolar vasoconstriction is more susceptible to damage by the irritating effects of meconium, so the pneumonia may be due the presence of meconium. However, if meconium gets into lungs that are otherwise healthy, the lung is unlikely to be adversely affected.