Overview
Clinical Definition and Mechanism
Meconium stained amniotic fluid (MSAF) means that fetal stool is present in the fluid at birth.
Meconium-stained amniotic fluid (MSAF) means that fetal stool is present in the fluid at birth. MSAF is an exposure, not a diagnosis. Meconium aspiration syndrome (MAS) is diagnosed when a newborn born through MSAF develops respiratory distress that cannot be explained by another condition. Meconium can obstruct the airways in two different ways. A complete obstruction prevents ventilation distal to the blockage and promotes atelectasis. A partial obstruction acts like a one-way valve: air enters during inspiration but cannot escape effectively during expiration. The trapped air hyperinflates the lung and can rupture into the pleural or mediastinal space, producing a pneumothorax or pneumomediastinum. The injury is not purely mechanical. Meconium disrupts surfactant function, so alveoli are harder to keep open. It also causes chemical pneumonitis, triggering inflammation and pulmonary vasoconstriction. The resulting ventilation-perfusion mismatch causes hypoxemia. Pulmonary vasoconstriction can produce persistent pulmonary hypertension of the newborn (PPHN), in which blood continues to bypass the lungs through right-to-left shunting. Post-term gestation and fetal compromise increase the likelihood of MSAF and MAS, but green fluid alone does not prove aspiration. Many newborns...