Inclusion criteria:
All preterm newborns with gestational age 28 -34 weeks, admitted to Imam Khomeini hospital, Ahvaz, 2020-2021
Exclusion criteria:
major congenital anomaly,
cyanotic congenital heart disease,
neuromuscular disease,
sepsis,
congenital pneumonia,
inborn errors of metabolism,
meconium aspiration syndrome,
perinatal asphyxia,
APGAR score=< 5 at 5 minutes,
intubation at birth or prolonged positive ventilation at the delivery room,
severe RDS necessitating intubation at birth based on NRP guidelines,
severe cardiovascular instability,
absence of spontaneous respiration and recurrent apnea,
no respiratory distress.