Meconium Aspiration Syndrome

Meconium is the newborn’s first intestinal stool. Passage into the amniotic fluid usually reflects in-utero or intrapartum stress, such as hypoxia or cord compression, and is more common in term and post-term pregnancies. Meconium-stained amniotic fluid occurs in roughly 10–15% of deliveries, but only a minority—about 3–5%—of exposed newborns develop meconium aspiration syndrome (MAS). MAS is therefore not synonymous with meconium-stained fluid, and it is rare before approximately 34 weeks’ gestation. Aspiration may occur before birth, during the first gasps, or with ineffective breathing after delivery. Once meconium enters the lower airways, it injures the lungs in several ways: - Thick material can partially plug an airway. Air enters around the plug during inspiration but cannot escape well during expiration, creating a ball-valve effect, air trapping, and hyperinflation. - Meconium inactivates surfactant, increasing alveolar collapse and the work required to breathe. - Direct chemical irritation produces pneumonitis and inflammation. - Pulmonary vasoconstriction increases pulmonary vascular resistance. Blood may continue bypassing the lungs through fetal circulatory pathways, producing persistent pulmonary hypertension of the newborn (PPHN) and severe hypoxemia. This mechanism explains...