IRCT201207182624N9 IRCT 2013-01-09 Vice chancellor for research , Tehran University of Medical Sciences. lowering labor pain Comparison between entonox and oxygen on lowering labor pain and the progress of labor 2011-03-21 anticipated 120 Complete https://irct.ir/trial/2414 interventional Randomization: Randomized, Blinding: Single blinded, Placebo: Used, Assignment: Parallel, Purpose: Treatment. 3 Delivery. Intervention 1: using entonox; the patients was taught to put the mask closely on their noses and breath continuously up to the end of uterine contraction and remove the mask between the contractions. During entonox using, it was temporarily discontinued in the occurrence of the side effects like nausea, vomiting, dizziness. This way was continued from cervical dilatation of 4-5 cm till full dilatation. Entonox is a mixture of 50% oxygen and 50% N2O. Intervention 2: using oxygen ; the patients was taught to put the mask closely on their noses as soon as contraction was begun and breath continuously up to the end of uterine contraction and remove the mask between the contractions. This way was continued from cervical dilatation of 4-5 cm till full dilatation. Oxygen is a 100% oxygen .