Respiratory distress syndrome (RDS) is potentially fatal in infants. The present study investigated the association of maternal serum vitamin D level with fetal pulmonary artery Doppler indices and neonatal RDS. This cross-sectional-analytical study was conducted on 260 mothers admitted for delivery. The maternal serum 25-hydroxyvitamin D level was measured and fetal main pulmonary artery Doppler indices were evaluated. The neonate's weight, apgar score, RDS, Umbilical cord arterial pH and neonatal intensive care unit admission were recorded. There was a significant relationship between RDS and the low level of vitamin D in the mother's serum. Mothers with low vitamin D had higher fetal pulmonary artery pulsatility index (PI) and peak systolic velocity (PSV) and lower acceleration time (AT) to ejection time (ET) ratio. Also, in neonates with RDS, pulmonary artery PI was higher, and PSV and the ratio of AT to ET were significantly lower than neonates without RDS. Neonates with lower one and 5-min apgar scores were born from mothers with lower vitamin D levels. Serum levels of vitamin D in pregnant mothers as well as fetal pulmonary artery Doppler indices are correlated to RDS. There is a significant relationship between the serum level of vitamin D in pregnant mothers and fetal pulmonary artery Doppler indices.