Strengths and weaknesses
The main strengths of the study were that it was large and contemporary.
This not only increases the generalizability of the findings, but also
reflects current practices in the US. In addition, while we quantified
the association between chronic hypertension and perinatal mortality, we
also performed a causal mediation analysis to evaluate the impact of
preterm delivery on this association. An application of causal inference
methodology in this context provides insight into what drives the risk,
which can be used to generate novel approaches to reduce the risks
associated with chronic hypertension.
Despite these strengths, the study as some limitations. There may be a
difference in risk for pregnancies complicated by mild chronic
hypertension versus pregnancies complicated by severe disease that
require antihypertensive medications, but we lacked data on the severity
of chronic hypertension. There is also possible misclassification bias
due to inaccuracy in recording diagnosis of chronic hypertension in the
data and inability to distinguish women who may have developed
superimposed preeclampsia. We have attempted to account for these
misclassification and unmeasured confounder biases in the sensitivity
analysis.