Blood pressure and hypertensive disorders of pregnancy at high altitude: a systematic review and meta-analysis

OBJECTIVE: Exposure to high altitude (>= 2500 m) is associated with increased arterial blood pressure. During pregnancy, even a mild elevation of maternal blood pressure is associated with reduced birthweight and increased prevalence of pregnancy complications. This study aimed to systematically assess the impact of altitude on maternal blood pressure at term and on the prevalence of hypertensive disorders of pregnancy. DATA SOURCES: PubMed, Ovid Embase, Cochrane Library, Medline, Web of Science, and ClinicalTrials.gov were searched (inception to November 11, 2020). STUDY APPRAISAL AND SYNTHESIS METHODS: Observational, cohort, or case-control studies were included if they reported a high-altitude and appropriate control pregnant population. Studies published >50 years ago were excluded; 2 reviewers independently assessed articles for eligibility and risk of bias. RESULTS: At high altitude, maternal systolic and diastolic blood pressure at term was higher than at low altitude (4.8 +/- 1.6 mm Hg; P<.001; 4.0 +/- 0.8 mm Hg; P<.001, respectively). Hypertensive disorders of pregnancy were more common at high altitude (odds ratio, 1.31 [1.03-1.65]; P<.05). The prevalence of gestational hypertension was nearly twice as high at high altitude (odds ratio, 1.92 [1.15-3.22]; P<.05) but the prevalence of preeclampsia was half as high (odds ratio, 0.57 [0.46- 0.70]; P<.001). The likelihood of stillbirth was increased by 63% in pregnancies at high altitude compared with low altitude (odds ratio, 1.63 [1.12 -2.35]; P<.01). CONCLUSION: Maternal blood pressure is higher at term in pregnancies at high altitude than low altitude, accompanied with an increased risk of gestational hypertension but not preeclampsia. Risk of stillbirth at high altitude is also increased. With a growing population residing at high altitude worldwide, it is essential to clearly define the associated risk of adverse pregnancy outcomes.