The main cause of respiratory distress syndrome (RDS) is a lack of surfactant in the lungs. Surfactant is a liquid that coats the inside of the lungs.
A fetus's lungs start making surfactant during the third trimester of pregnancy (weeks 26 through labor and delivery). The substance coats the insides of the air sacs in the lungs. This helps keep the lungs open so breathing can occur after birth.
Without enough surfactant, the lungs will likely collapse when the infant exhales (breathes out). The infant then has to work harder to breathe. He or she might not be able to get enough oxygen to support the body's organs.
Some full-term infants develop RDS because they have faulty genes that affect how their bodies make surfactant.