Since in our setting, only preterm pregnant women of 24 0/7 to 33 6/7 weeks of gestation would receive a course of antenatal corticosteroid administration for accelerate fetal lung maturation. So the success of preterm uterine contraction inhibition was assessed as the delay in delivery for at least 48 hours after tocolytics administration between 24 and 33 week of gestation while the inhibition failure was defined as the delay in delivery for ,48 hours. The risk factors of the inhibition failure were also analyzed in this study.
The sample size calculation based on the objective to assess the success rate of preterm uterine contraction inhibition with the maximal proportion of defined women was 0.5% and 10% of allowance error. The study required at least 96 singleton pregnant women who admitted to the labor room with preterm uterine contraction and received tocolytic agents for inhibition.