Interpretation
An understanding of the perinatal risks associated with chronic
hypertension is largely informed by older studies. For example, a prior
population based study from the US, which utilized Nationwide Inpatient
Sample data from 1995-2008, reported a higher risk for stillbirth (odds
ratio [OR] 2.31, 95% CI 2.11, 2.53) for pregnancies complicated by
chronic hypertension compared to normotensive pregnancies.(6) In a
population‐based prospective cohort study from Sweden (1992-2004), an
increase in stillbirth risk (OR 2.71, 95% CI 1.96, 3.73) and neonatal
death (OR 2.89, 95% CI 1.95, 4.30) was observed.(21) The results of our
study, which showed increased perinatal mortality associated with
chronic hypertension, are consistent with these studies, yet the
magnitude of risk was larger. After adjusting for bias due to
misclassification and unmeasured confounders, we found the risk of
perinatal mortality was 4.86 (95% CI 3.42, 5.98), which was largely due
to stillbirth at term gestations. The reason for this difference is
uncertain, but may be due to advances in neonatal care that minimize the
risk of neonatal deaths.
The mediation analysis suggested that a substantial driver of perinatal
mortality risk is preterm delivery. Although most deliveries in our
cohort occurred at term, there was an association between chronic
hypertension and preterm delivery. A prior population based study from
the US found the odds of preterm delivery for chronic hypertensive
versus normotensive pregnancies was 3.01 (95% CI 2.88, 3.14).(6) A
meta-analysis on chronic hypertension and pregnancy outcomes showed that
pregnancies complicated by chronic hypertension are at a higher risk of
preterm delivery <37 weeks’ gestation (RR 2.7, 95% CI 1.9,
36).(8) Most often the preterm births are medically indicated secondary
to superimposed preeclampsia or fetal growth restriction.(40, 41) This
study suggests that, if possible, avoiding preterm delivery in
pregnancies complicated by chronic hypertension may have large impact on
perinatal mortality.
The results of our study are consistent with prior work(42) and
professional society recommendations(43) that suggest the optimal timing
of delivery for pregnancies complicated by chronic hypertension is 37-38
weeks. It is critical to avoid early preterm delivery in these
pregnancies (due to preterm delivery-associated complications and
long-term chronic health conditions), but term and late term pregnancies
also carry high risk of stillbirth. Despite national efforts to avoid
early term deliveries,(44) findings from this study suggest early term
deliveries may minimize the risks of stillbirths associated with chronic
hypertension.
Women with chronic hypertension often have risk factors, such as
advanced maternal age, obesity, and tobacco smoking,(1, 45, 46) which,
in turn, increase the risks of adverse perinatal outcomes.(47-51) Public
health strategies that target modifiable risk factors to reduce the
burden of chronic hypertension may be the best strategy to reduce
perinatal mortality associated with this disease. The mediation analysis
quantified the preventable proportion of perinatal deaths associated
with chronic hypertension by eliminating preterm deliveries associated
with chronic hypertension, and most of perinatal deaths could be
avoided.
While this study improves our understanding of the magnitude of chronic
hypertension’s impact on perinatal mortality, there is a need for
further study: (i) to develop preventive strategies such as weight loss
and preconception blood pressure management for women at risk; (ii) to
assess the effectiveness of these strategies on perinatal outcomes; and
(iii) to evaluate whether treating mild chronic hypertension in
pregnancy has the potential to reduce the risk of adverse pregnancy
outcomes.(52)