Heat beyond percentiles: exploring preterm birth risks in Santiago, Chile (1991-2019).

OBJECTIVE

Evaluate the relationship between extreme heat in the week and month before delivery and risk of preterm birth (< 37 weeks’ gestation) using secondary data from Santiago, Chile.

METHODS

We conducted a population-based cohort design using secondary information from birth records (1991–2019) and limited analysis to births in spring/summer months (n = 957,734). Temperature percentiles and the excess heat factor (EHF) were calculated for each of the 33 urban municipalities in Santiago using historical data. We derived 15 distinct heatwave metrics by combining three durations (2, 3, and 4 consecutive days) with five temperature thresholds: temperatures > 30 °C, the 90th, 95th, and 99th percentiles, and EHF. For each municipality, exposure (Yes or No) was assigned for each of these metrics in the week and month prior to delivery. We estimated Cox proportional hazard models adjusted for known confounders.

RESULTS

Preterm birth occurred in 6% of births. There was spatial variability in extreme temperatures for the municipalities. Exposure to most heat wave metrics (≥ 2, ≥ 3, ≥ 4 days > 90th, 95th, and 99th percentile), did not relate to increased risk of preterm birth. Exposure to EHF for ≥ 2, ≥ 3, ≥ 4 days was associated with 1.02 (95% CI 1.00–1.04), 1.03 (95% CI 1.01–1.04), and 1.03 (95% CI 1.02–1.05), respectively. Exposure to ≥ 2, ≥ 3, ≥ 4 days of temperatures > 30 °C were associated with decreased hazards.

CONCLUSION

We found evidence of increased risk of preterm birth, but only when evaluating exposure using EHF, a metric that considers acclimatization. Our results highlight the importance of understanding regional impacts of climate change on child health.