Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Respiratory Distress Syndrome ?
    ### Response:
    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).
    