Statistical analysis
First, we studied trends over time in indicators of birth asphyxia. We
calculated the number and rate of births with the outcome measurements
for each year of the study. To analyse possible trends over time, we
performed Cochrane-Armitage trend tests. The absolute difference was the
outcome in year 2019 minus 2010. The relative difference (in percentage)
was the absolute difference divided by the outcome in year 2010.
Second, we investigated risk factors for a low 5-minute Apgar score,
both <7 and <4, by calculating risk per 100 births
and performing logistic regression analysis. We assessed crude and
adjusted odds ratios (ORs) with 95% confidence intervals (CI). The
Population Attributable Risk (PAR), which takes the number of exposed
individuals into account, was calculated for the variables with
statistically significant ORs for Apgar score, both <7 and
<4.
Thirdly, we studied trends in prespecified obstetric interventions
(induction of labour, use of epidural analgesia, planned and emergency
caesarean section, and instrumental delivery) and in level of care. In
subgroups based on these obstetric interventions, we also separately
assessed trends in low Apgar scores.
Finally, we performed a multivariate logistic regression analysis to
adjust for the co-variates. Collinearity diagnostics was performed to
find possible interaction of factors. Data of confounders were missing
in less than 0.5% of the cases, except for arterial umbilical cord
blood pH’s. Arterial umbilical cord blood pH’s are not routinely
collected and were available in 9.6% of all infants. Missing data of
co-variates were imputed. All analyses were complete case analyses. All
statistical analyses were carried out with SAS software version 9.4 (SAS
Institute Cary, North Carolina USA). A p- value of less than 0.05
was considered statistically significant.