With less than 200,000 cases per year, Neonatal Respiratory Distress Syndrome (or Neonatal RDS) occurs when a child’s lungs have not fully developed and are lacking the substance surfactant, which helps the lungs fill with air and keeps the sacs from deflating. This usually occurs in infants born before 37 to 39 weeks.
Here are two websites with more information:
U.S. National Library of Medicine
Healthline