Urban Heat Exposure and Adverse Pregnancy Outcomes in Low- and Middle-Income Countries: A Systematic Evidence Synthesis

Abstract

Background: Rising ambient temperatures and more frequent heat waves may increase pregnancy risk, particularly in low- and middle-income countries (LMICs) where baseline rates of preterm birth, stillbirth, low birth weight, outdoor work, poor housing, limited cooling, and constrained maternity services can coincide. Although urban heat islands are an important exposure pathway, pre-2024 LMIC evidence also demonstrates substantial rural and occupational vulnerability.
Objective: To synthesize Q1 evidence available through 31 December 2023 on heat exposure and adverse pregnancy outcomes in LMICs, with emphasis on preterm birth, stillbirth, low birth weight, hypertensive disorders, fetal strain, urban-rural differences, and susceptibility windows.
Methods: A structured systematic evidence synthesis was conducted using Q1 studies in obstetrics, environmental health, planetary health, and general medicine. Direct LMIC cohort, case-crossover, time-series, and physiological studies were prioritized, supported by high-quality systematic reviews. A new pooled effect estimate was not calculated because exposure metrics, temperature contrasts, lag periods, and outcomes differed substantially.
Results: The 2020 BMJ meta-analysis found 5% higher odds of preterm birth per 1°C increase in temperature and 16% higher odds during heat waves; all eight stillbirth studies showed harmful associations, with an approximately 5% increase per 1°C. A 2022 analysis across 14 LMICs linked acute late-pregnancy heat with both preterm birth and stillbirth. In 126,273 pregnancies from India and Pakistan, second-trimester temperature was associated with preterm birth (RR 1.05, 95% CI 1.02–1.07) and low birth weight (RR 1.02, 95% CI 1.01–1.04), while third-trimester temperature was associated with gestational hypertensive disease (RR 1.07, 95% CI 1.02–1.12). Ghanaian national data showed up to 18% higher stillbirth risk at higher-moderate long-term thermal stress, and Chinese national data showed increased preterm-birth risk after compound and daytime-only heat waves, particularly among rural women. A field study in The Gambia demonstrated acute maternal heat strain and fetal strain during real-world heat exposure.
Conclusion: Heat exposure is a credible and increasingly important risk factor for adverse pregnancy outcomes in LMICs. Risk is shaped not only by ambient temperature but by timing, duration, nighttime recovery, occupation, housing, access to cooling and water, nutrition, rurality, and health-system resilience. Adaptation strategies should therefore combine heat-warning systems, occupational protection, maternity counseling, cooling and hydration access, and locally validated temperature thresholds.