Statistical analysis
Sample size calculation was performed by using data from van Bavel’s study, which evaluates the risk of OASI: the true risk of OASI in primiparous women with mediolateral episiotomy is 3% and 26 % in instrumental delivery.4 A sample size of 33 for each group was calculated based on these data and a one-sided t -test (p < 0.05) was estimated to have 80% power to reject the null hypothesis that uterine fundal pressure does not increase the risk of OASI as much as forceps delivery. Statistical analysis was performed after normality testing (histogram analysis and/or Kolmogorov-Smirnov test) using IBM SPSS, version 21 (IBM Inc., Armonk, NY). Student’s t-test was used for the comparison of normally distributed variables, while the Mann-Whitney U-test was used for non-parametric variables, and Fisher’s exact tests were used to compare categorical variables. Variables were then considered jointly in a multivariable stepwise forward multivariable logistic regression model to predict OASI.