Results
During the study period, 197,185 deliveries were recorded; 140,855 (71.4%) deliveries met the study inclusion criteria (Figure 1 ). A total of 159 CNMs worked during the study period at the SZMC, attended at least one delivery annually and were included in the study. The median annual vaginal births attended, during the study period was 152 deliveries [114-195]; 70,849 deliveries (50.3%) were assigned to the ”low group” and 70,006 deliveries (49.7%) to the ”high group” in accordance with the CNM ”low” and ”high” annual volume, respectively.
Per definition, there was a difference in the number of deliveries attended by the CNM in the two groups; the ”low volume” group was delivered by CNMs with an average of 108.3±30.5 deliveries annually and the ”high volume” group was delivered by CNMs with an average of 208.2±49.6 deliveries annually. The ”low volume” group was delivered by CNMs with a longer mean practice duration as compared with the ”high volume” group (12.6±10.7 vs 7.5±8.2 years, p<0.001).
Several statistical significant differences were noted in maternal characteristics between groups and these are detailed in Table 1 . These differences were noted in the following variables (low volume vs. high volume): higher maternal age, higher gravidity, higher parity, higher rate of previous CD, advanced gestational age at delivery, higher rates of diabetes mellitus, lower rate of augmentation of labor, lower rate of persistent occiput posterior and shorter duration of second stage. The incidence of previous miscarriages, hypertensive pregnancy disorders, fertility treatments, induction of labor, epidural analgesia, chorioamnionitis as well as prolonged second stage did not differ significantly between groups.