Discussion
Main findings: In this retrospective study, we examined the association between the CNM’s annual workload and short-term maternal and neonatal outcomes of term singleton vaginal vertex deliveries. Only few minor differences in maternal and neonatal outcomes were noted between those delivered by CNM with low workload as opposed to those with high workload. Even though univariate analysis revealed higher rates of some maternal outcomes (composite adverse maternal outcomes, spontaneous perineal tears both 1st and 2nd degrees, uterine manual revision for suspected retained placental products and lower rates of episiotomies) on multivariate analysis these differences were no longer apparent. However, some neonatal outcomes differed between CNM with high and low annual delivery workload both using univariate analysis and multiple regression analysis. Low CNM annual delivery workload was associated with higher composite adverse neonatal outcome, neonatal jaundice and mechanical ventilation rates. Further analysis in deciles revealed similarly that there is no reduction in maternal or neonatal morbidity in higher deciles and that the findings reported for the entire population valid for somewhat higher risk population – the nulliparous population.
Despite the statistically significant statistics in specified outcomes, the relatively low odds ratio, the fact that confidence intervals were close to one and the lack of difference in other analysis such as deciles or specific higher risk groups such as primiparity led us to think that these noted differences could be simply a reflection of our relatively large sample size and thus lacks major clinical significance.Interpretation: The association between an individual practitioner and / or medical center annual volume rates of surgical procedures, morbidity and mortality had been well established in previous studies13,14. Annual procedure volume is an accepted quality marker and has been shown to correlate directly with morbidity and mortality rates in surgical and high-risk medical procedures4,14–16. It has also been suggested that volume outcome relationship is not always linear and that, in some instances, there appears to be a threshold which differs according to the procedure13.
In the current study, rates of spontaneous 1st (26.4% vs. 25.6%, p=0.05) and 2nd (26.9% vs. 25.8%, p=0.01) degree perineal tears and overall perineal tears (32.3% vs. 31.2%, p<0.001) were higher among parturient delivered by the low annual volume group. These rates are parallel with what has been recently published in a systematic review and meta-analysis of rates of birth-related perineal trauma17. These findings are also in accordance with a previous studies that showed an association between overall perineal tears and 1st and 2nd degree perineal tears and midwife experience18,19. In our study rates of 3rd and 4th degree perineal tears did not differ between the groups. These rates are similar to what has been previously published, however in one study there was an association between all 4 degrees of perineal tears and the midwife’s experience19.
With regard to neonatal outcomes, the statically significant higher rates of the composite adverse neonatal outcome among the low annual volume group (10.9% vs. 10.3%, p<0.001%) was attributed to jaundice (4.1% vs. 3.7%, p<0.001%) and mechanical ventilation (0.3% vs. 0.2%, p<0.001). These rates were comparable with a large Norwegian population-based study that assessed adverse perinatal outcomes in 665,244 term and post-term deliveries20
Even with the statistically significant difference in the mechanical ventilation rate (0.3% vs. 0.2%) no other statistically significant difference in Apgar scores, neonatal asphyxia and NICU Admissions rates were determined.
In the planning of the current study, we anticipated that higher individual CNM delivery volume would translate into better perinatal outcome, however, we have noted only some statistically significant differences. The absolute magnitude of these differences was minor and likely without clinically significant meaning. We believe that this may be attributed to several factors.
Primarily, labor and delivery is a natural process, and when compared to CD, spontaneous vaginal delivery is associated with decreased maternal morbidity21. Even in the surgical field, in some procedures such as rectal cancer surgery, surgeon volume appears to have no effect or a small beneficial effect on complication rates, and survival22. Furthermore, the CNM’s extensive training program and the ”shared model” of care applied in our setting may be related to the favorable outcome of the low annual volume group. Using the shared model of care enables the on duty CNM controller (an experienced CNM who oversees the entire CNM team working in her shift) together with an attending obstetrician to oversee all deliveries and CNM work. As a result, each delivery is objectively observed by several independent care givers, allowing for better teamwork, nursing and medical care, shared thinking, decision making and responsibility. This model eliminates, at least in part, the option of preventable medical mistakes, ”near misses” during labor and facilitates proper delivery management. Finally, it is possible that most of adverse outcomes seen during labor and delivery are a direct result of the inherent maternal and fetal characteristics (i.e neonatal macrosomia, maternal BMI etc.) and little may be attributed to inexperience CNM techniques2.
Strengths and imitations: Our study possesses several strengths. Predominantly, to the best of our knowledge this is the first study that examines the association between CNM’s annual volume and various maternal and neonatal outcomes. It is based on a large sample size of deliveries and of CNMs. All records used in this study were derived from real time updated computerized database, minimizing the possibility of bias. In addition, our study outcomes rates, both maternal and neonatal, are on par with what is reported in the literature.
This study has various limitations; We included only spontaneous term vaginal deliveries, thus the association between the CNM volume and VAVD/CD cannot be concluded from the current study2. Additionally, our definition of low vs. high annual volume was arbitrary and chosen as the median annual volume. However, further deciles analysis has shown the same pattern without any evident threshold for better outcomes. Lastly, it is possible that assessing single CNM experience in a large academic medical center may not reflect volume/outcome relationships in other centers.
Conclusion: In low risk term vaginal deliveries, perinatal outcomes were mostly not affected by CNM’s annual workload. These findings suggest that having term singleton vaginal deliveries delivered by a low volume CNM is as safe as delivering with a higher annual volume CNM. Future studies should assess the effect of CNM’s annual workload in various setups.
Acknowledgments: NoneDiscloser of interests: None