Statistical analyses
We estimated perinatal mortality rates, expressed per 1000 births, for
chronic hypertensive versus normotensive pregnancies. We examined the
association between chronic hypertension and perinatal mortality on both
the additive (risk difference, RD) and multiplicative scales (risk
ratio, RR), derived from fitting log-linear Poisson regression models
with robust variance and the identity-link for RD and log-link for RR
(29). Associations with their corresponding 95% confidence intervals
(CI) were derived before and after adjusting for confounders. We also
examined associations between chronic hypertension and the risks of
stillbirth, early neonatal deaths, and late neonatal deaths.