High ambient temperatures and hypertensive disorders of pregnancy: a systematic review and meta-analysis protocol.

Background: Exposure to high ambient temperatures, commonly referred to as heat, has been consistently associated with adverse birth outcomes, particularly preterm birth. There is less consensus with regard to links with adverse maternal outcomes, such as hypertensive disorders of pregnancy (HDP). HDP are a leading cause of maternal mortality and morbidity, whose aetiology remains poorly understood. In the context of global warming driven by climate change, clarifying this potential association is of importance for global health. Objective: The proposed systematic review and meta-analysis aims to synthesise and critically appraise the available peer-reviewed literature on heat exposure and HDP, with specific attention paid to differences across HDP subtypes, windows of susceptibility and study methods. The findings will provide novel insights into the potential role of heat in the aetiology of HDP, and guide future research. Methods: A systematic search of PubMed, EMBASE and Web of Science will be undertaken using the detailed prespecified search strategy that was reviewed by an expert librarian. Additional texts will be identified through snowball reference searching. Two authors will independently screen titles, abstracts and full-texts, and undertake data extraction, with discrepancies resolved by a third reviewer. HDP are the primary outcomes of interest and include gestational hypertension, pre-eclampsia (de novo, superimposed, early- and late-onset), and associated complications, namely eclampsia and haemolysis, elevated liver enzymes and low platelet count (HELLP) syndrome. Secondary considerations include identifying windows of susceptibility, evaluating epidemiological designs and statistical methods employed. Findings will be summarised through narrative synthesis and quantitatively through random-effects meta-analyses where possible. Risk of bias in individual studies and confidence in the cumulative body of evidence will be assessed following the Navigation Guide Criteria. The review will be reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses and the Meta-analyses Of Observational Studies in Epidemiology guidelines.