Severe Maternal Morbidity Associated With Chronic Hypertension, Preeclampsia, and Gestational Hypertension.

Importance: Chronic hypertension and preeclampsia are leading risk enhancers for maternal-neonatal morbidity and mortality. Severe maternal morbidity (SMM) indicators include heart, kidney, and liver disease, but studies have not excluded patients with preexisting diseases that define SMM. Thus, SMM risks for uncomplicated chronic hypertension specific to preeclampsia remain unclear. Objective: To determine SMM rates and estimate relative risks associated with hypertensive disorders of pregnancy among patients with and without chronic hypertension unencumbered by preexisting vascular or end organ diseases. Design, Setting, and Participants: This retrospective cohort study used longitudinal health data from electronic health records from patients within a community-based, integrated health care system in northern California. The study cohort selected 263 518 pregnant patients without pregestational heart, kidney, or liver disease entering prenatal care at 14 weeks' gestation or earlier and delivering a singleton stillbirth or live birth in 2009 to 2019. The data were analyzed between February 2022 and March 2024. Exposures: Five joint subgroups combining chronic hypertension status and the hypertensive disorders developing during pregnancy, defined as follows: (1) chronic hypertension with superimposed preeclampsia, (2) chronic hypertension and no preeclampsia, (3) no chronic hypertension with preeclampsia, (4) gestational hypertension, and (5) no chronic hypertension and no preeclampsia or gestational hypertension (reference group). Main Outcomes and Measures: The main outcome was SMM rates at delivery hospitalization (cases per 10 000 births) using the Centers for Disease Control and Prevention criteria (≥1 of 21 indicators to define SMM) obtained from electronic health records. Modified Poisson regression models estimated crude and adjusted relative risks (aRRs) and 95% CIs of SMM associated with the chronic hypertension and developing hypertensive disorders of pregnancy groups vs the reference group (no chronic hypertension and no preeclampsia or gestational hypertension) adjusted for clinical, sociodemographic, social, and behavioral covariates. Results: The analysis included a total of 263 518 pregnant patients (mean [SD] age at delivery, 31.0 [5.3] years), including 249 892 patients without chronic hypertension (4.7% developed preeclampsia) and 13 626 patients with chronic hypertension (31.5% developed superimposed preeclampsia). The highest SMM rates occurred in the no chronic hypertension with preeclampsia (934.3 [95% CI, 882.3-988.3] cases per 10 000 births) and the chronic hypertension with superimposed preeclampsia (898.3 [95% CI, 814.5-987.8] cases per 10,000 births) groups. Lower SMM rates occurred in the chronic hypertension and no preeclampsia (195.1 [95% CI, 168.0-225.2] cases per 10,000 births), gestational hypertension (312.7 [95% CI, 281.6-346.1] cases per 10,000 births), and no chronic hypertension and no preeclampsia or gestational hypertension (165.8 [95% CI, 160.6-171.2] cases per 10,000 births) groups (P < .001). Compared with the no chronic hypertension and no preeclampsia or gestational hypertension group, risks of SMM were significantly higher for the chronic hypertension with superimposed preeclampsia group (aRR, 4.97 [95% CI, 4.46-5.54]), no chronic hypertension with preeclampsia group (aRR, 5.12 [95% CI, 4.79-5.48]), chronic hypertension and no preeclampsia group (aRR, 1.17 [95% CI, 1.003-1.36]; P = .046), and the gestational hypertension group (aRR, 1.78 [95% CI 1.60-1.99]). Conclusions and Relevance: This cohort study found that the highest SMM rates at delivery hospitalization occurred for preeclampsia superimposed on chronic hypertension and preeclampsia without chronic hypertension, while gestational hypertension had intermediate rates of SMM. The patients with chronic hypertension who did not develop preeclampsia had SMM rates that were nearly the same as the lowest-risk patients without chronic hypertension who did not develop preeclampsia or gestational hypertension. These findings provide evidence that prevention of preeclampsia among patients with uncomplicated chronic hypertension is paramount to mitigating maternal morbidity.