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