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.