TTN is most commonly seen in which infant group, and what mechanism underlies it?

• A. Term or late preterm infants; delayed clearance of fetal lung fluid.
• B. Very preterm infants; surfactant deficiency.
• C. Post-term infants; pulmonary edema.
• D. Term infants with diaphragmatic hernia; airway compression.

Answer: (A)

Transient tachypnea of the newborn most commonly occurs in babies born at term or late preterm. The key mechanism is delayed clearance of fetal lung fluid after birth. Normally, labor triggers hormonal and mechanical cues that help squeeze fluid out of the lungs and activate the pumps that reabsorb remaining fluid. When this clearance is slower than usual—often after cesarean delivery without labor—the residual fluid in the airways and interstitium leads to transient tachypnea and mild respiratory distress that typically improves within a day or two with supportive care. This pattern distinguishes TTN from conditions like surfactant deficiency in very preterm infants, which has a different underlying cause and timing.