Statistical analysis
For each variable (demographic, in-labour and delivery) differences
across delivery groups (1-5) were compared using either ANOVA or
Kruskal-Wallace ANOVA for normal or non-normally distributed data
respectively. 95% confidence intervals and odds ratios were calculated
using univariate logistic regression analysis to examine 1) the
association between instrumental delivery mode (groups 1-4) and
spontaneous vaginal delivery, and 2) the relationship between all
delivery modes (groups 1-5) and overall composite risk outcome.
To adjust for baseline risk, a stepwise, multivariate regression
analysis was performed. Indices were included based on clinical
plausibility and/ or a significant association with the following
dependent variables in univariate analysis: successful vaginal delivery,
maternal composite risk outcome, neonatal composite risk outcome and
overall composite risk outcome. These were inputted alongside delivery
mode to produce two final multivariate binomial regression models: one
with a dependent variable of successful vaginal delivery (delivery
groups 1-4) and another with a dependent variable of overall composite
risk outcome (delivery groups 1-5). Receiver operating characteristic
curve analysis was performed on these final models with an area under
the curve (AUC) of 0.80 considered to represent reasonable prediction.
All data was collected into Microsoft Excel and analysis performed using
IBM SPSS statistical software Version 26.
Results
Figure 1 displays the breakdown of delivery types occurring at the
hospital over the study period (2014-2018). Of 24,756 deliveries 66.5%
were spontaneous vaginal deliveries, 22.48% were Caesarean deliveries
and 10.6% were instrumental deliveries. From the 2631 instrumental
deliveries performed over the study period, 991 (37.6%) were performed
in obstetric theatre as a trial of instrumental delivery. Excluding
unavailable or twin pregnancy data (20 datasets), remaining cases were
organised into 5 groups according to the initial delivery attempt from
which 285 (29.3%) were KFD, 300 (30.8%) were DFD, 163 (16.7%) were
MR+FD, 116 (11.9%) were VD, and 107 (11%) were pEmCS.
Across groups 1 to 5, the data collected for demographic variables is
displayed in Table 1. Differences between groups were observed for both
body mass index (BMI) and weight, with the pEmCS group having a
significantly higher mean average weight. As height did not vary
significantly between groups, this led to a weight-dependent difference
in BMI between groups. This factor was accounted for with the inclusion
of BMI as a final variable in the multivariate models described below.
The data representing all in-labour and delivery variables can be
observed in Supplementary Table 1. There were a greater proportion of
multiparous women in both the KFD and DFD groups compared with pEmCS;
the deliverer was more often a doctor of greater experience undertaking
the Keilland’s deliveries compared with other groups and pEmCS were
significantly more likely than other groups to be performed for fetal
distress and to be performed under general anaesthetic. The direct
forceps delivery group had a greater proportion of occipito-anterior
position and low cavity (fetal station 2+ and below) deliveries than
other groups. Use of 2 instruments was greater in the ventouse delivery
group compared with other instrumental delivery groups.
The primary outcome of vaginal delivery was investigated initially via
univariate regression analysis, with the instrumental delivery groups
(1-4) compared, using group 1 (KFD) as the reference (Figure 2A).
Attempt at manual rotation and direct forceps was associated with the
greatest percentage of completed vaginal delivery (92%), with direct
forceps (89.7%) and Keilland’s (83.2%) second and third respectively.
Attempt at ventouse delivery displayed the lowest percentage completion
rate for vaginal delivery at 75%, consistent with ventouse delivery
completion rates seen in existing literature 15. When
compared with the reference group of Keilland’s forceps, raw odds ratios
suggested that use of direct forceps delivery (OR 1.75, 95%CI: 1.08 –
2.05) and manual rotation plus forceps delivery (OR 2.33, 95%CI:
1.22-4.45) might increase chances of vaginal delivery; with ventouse
delivery less likely to achieve vaginal delivery (OR 0.60, 95%CI:
0.36-1.02). A stepwise, multivariate regression analysis was performed
to further investigate this trend and adjust for baseline confounding
and risk factors. The final model adjusted for maternal BMI, birth
weight, parity, analgesia, experience of deliverer, fetal position and
fetal station. Ventouse delivery was significantly less likely to
succeed at vaginal delivery when compared to KFD in this adjusted model
(OR 0.426, 95%CI 0.227-0.797). To assess the robustness of predictive
value for this model a receiver operator characteristic curve was
produced (Supplementary Figure 1A) which demonstrated an area under the
curve of 0.768.
The data demonstrating immediate perinatal adverse outcomes are
presented in Table 2. There were two neonatal deaths in the cohort, one
in the Keilland’s group and one in the ventouse group, there were no
significant neonatal injuries noted in any groups. Of note, instrumental
deliveries other than Keilland’s forceps were associated with higher
rates of 3rd and 4th degree tears,
with the highest proportion in the ventouse group at 10.3%. Primary
emergency full dilatation Caesarean was associated with the highest
proportion of maternal injuries during Caesarean (8.4%) and a greater
frequency of babies with Apgar score of <7 at 5 minutes
(9.3%), the latter possibly associated with a much higher proportion of
these deliveries being performed under general anaesthetic (22.4%) than
was observed in the other delivery groups (Supplementary Table 1).
The secondary outcome of overall composite risk score was investigated
via univariate regression analysis, with the all delivery groups (1-5)
compared, using group 5 (pEmCS) as the reference group, the results of
which are displayed in Figure 2B. An outcome associated with immediate
risk occurred in 22.8% of all deliveries in the cohort, indicating the
high-risk nature of such full dilatation delivery trials. Keilland’s
forceps deliveries were associated with the lowest proportion of
composite risk outcome occurrence (17.5%), and ventouse deliveries the
highest (29.3%). When compared with the reference group of pEmCS, there
were no significant associations between mode of delivery and composite
risk outcome. This analysis was further modified using multivariate
logistic regression to adjust for baseline risk factors. The final
variables included in this model were: maternal BMI, birth weight,
analgesia, experience of trial decision maker, indication for trial,
experience of deliverer, fetal position and fetal station. The adjusted
model demonstrated significant differences (P=0.016) for composite risk
outcome occurrence between the pEmCS and KFD delivery groups (OR 0.37,
95% CI: 0.16-0.81), however the receiver operating characteristic curve
produced from this model demonstrated low predictive value with an area
under the curve of 0.64 (Supplementary Figure 1B).
Discussion