Overview
What the Meconium Does to the Lung
Meconium is the newborn’s first stool. When it appears in the amniotic fluid, it often reflects fetal or intrapartum stress such as hypoxemia or cord compression, particularly i...
Meconium is the newborn’s first stool. When it appears in the amniotic fluid, it often reflects fetal or intrapartum stress such as hypoxemia or cord compression, particularly in term and post-term pregnancies. Meconium-stained amniotic fluid is common—roughly 10–15% of deliveries—but exposure alone does not mean the infant has meconium aspiration syndrome (MAS). Only a minority of exposed infants develop respiratory disease. MAS occurs when the infant inhales meconium before, during, or immediately after birth. The material obstructs smaller airways. Partial obstruction can act like a one-way valve: air enters during inspiration but cannot leave effectively during expiration, producing air trapping and hyperinflation. A complete obstruction causes atelectasis distal to the blockage. Meconium also inactivates surfactant, so alveoli become more difficult to keep open. The injured lung develops chemical pneumonitis, which increases inflammation and impairs gas exchange. Pulmonary vasoconstriction may then raise pulmonary vascular resistance and sustain fetal-type right-to-left blood flow through the ductus arteriosus or foramen ovale. This is persistent pulmonary hypertension of the newborn (PPHN), and it can produce severe, labile hypoxemia even when the infant is receiving substantial...