Statistical analysis:
Univariate analysis was performed for categorical variables with the use of the χ2 or Fisher’s exact tests. Continuous variables were compared with the use of the Student t  test or Mann-Whitney U  test. To evaluate the independent association between CNM annual volume and maternal outcomes, we conducted multivariate logistic regression modeling, adjusting for patient demographic parameters and comorbidities: maternal age, parity, gestational age, trial of labor after cesarean delivery (TOLAC), second stage duration, persistent occipito-posterior, 2nd degree perineal tear and episiotomy. Additional analysis was performed to assess whether association exists between CNM’s annual volume and each neonatal outcome adjusting for: maternal age, parity, gestational age, TOLAC and second stage duration. All tests are 2-tailed; a probability value below .05 was considered statistically significant. Analyses were carried out with SPSS software package (version 25.0; IBM Corporation, Armonk, NY).
The study was approved by the local Institutional Review Board of SZMC (IRB-0044-20, 15.08.2108) and was exempted from obtaining informed consent as the study was based on patient records.