It is always considered to shorten the duration of delivery without compromising the mother and fetus by obstetrics. Hyoscine and Promethazine are used widely in hospital and research is very limited, with conflicting results regarding the efficacy of these two drugs. So the investigators decided to compare of effect of hyoscine N-butyl bromide and Promethazine on length of active phase of the first stage of labor.

Inclusion Criteria:

• 18-35 yrs,
• primiparous,
• having single fetus with cephalic presentation, gestational age 37-42 weeks,
• having no medical diseases,
• having no history of hospitalization due to psychological diseases,
• having no addiction to cigarette,
• having no complications during pregnancy such as hypertension,
• BMI< 26

Exclusion Criteria:

• bleeding
• CPD
• use of PG or Oxytocin
• fetus distress