Ambient temperature exposure and hypertensive disorders of pregnancy in Sweden

Background

Hypertensive disorders of pregnancy (HDP) are major causes of maternal morbidity and mortality. Evidence suggests ambient temperature may influence HDP risk, but existing studies are limited by small samples, inconsistent methods, and unclear exposure windows. We examined associations between extreme temperature during pregnancy and HDP risk, and identified critical exposure windows.

Methods

We included pregnancies from the Swedish Pregnancy Register (2014–2024). Daily ERA5 air temperature data (0.25°×0.25°) were linked to residential addresses. Weekly mean temperature exposure was evaluated across gestational weeks 0–19, as HDP are defined after 20 weeks, and within consecutive four-week intervals to assess timing, particularly during the placentation period. Gestational hypertension and preeclampsia were identified using ICD-10 codes. Logistic regression with distributed lag non-linear models estimated associations with extreme heat and cold, adjusting for adjusting for key covariates.

Results

Among 1,069,892 pregnancies, 37,421 (3.5%) had gestational hypertension and 33,878 (3.2%) preeclampsia. Heat exposure during weeks 0–19 (95th vs 75th percentile; 18.4 °C vs 14.0 °C) was associated with lower odds of gestational hypertension (adjusted odds ratio [aOR] 0.85, 95% CI 0.76–0.95), particularly during weeks 4–15. Cold exposure (5th vs 25th percentile; −3.7 °C vs 2.1 °C) was associated with higher odds (aOR 1.07, 95% CI 1.01–1.14), mainly during weeks 16–19. No significant associations were found for preeclampsia (heat aOR 0.98, 95% CI 0.88–1.10; cold aOR 1.00, 95% CI 0.94–1.07).

Conclusions

Ambient temperature exposure during early pregnancy was differentially associated with gestational hypertension, with heat linked to lower odds and cold linked to higher odds. No associations were observed for progression to preeclampsia. Temperature alerts in antenatal care guide protective prenatal care measures during extreme weather.

Key messages

• Ambient heat and cold exposure during pregnancy are associated with gestational hypertension but not preeclampsia, with specific critical gestational windows identified.

• Findings support integrating temperature-related risk awareness into antenatal care to better protect pregnant individuals during extreme weather periods.