Exposure to Ambient Heat and Risk of Spontaneous Abortion: A Case–Crossover Study

Abstract

Background: Few epidemiologic studies have examined the association of ambient heat with spontaneous abortion, a common and devastating pregnancy outcome. Methods: We conducted a case–crossover study nested within Pregnancy Study Online, a preconception cohort study (2013-2022). We included all participants reporting spontaneous abortion (n=1,524). We defined the case window as the 7 days preceding the event and used time-stratified referent selection to select control windows matched on calendar month and day of week. Within each 7-day case and control window, we measured mean, maximum, and minimum of daily maximum outdoor air temperatures. We fit splines to examine non-linear relationships across the entire year and conditional logistic regression to estimate odds ratios (OR) and 95% CI of spontaneous abortion with increases in temperature during the warm season (May-September) and decreases during the cool season (November-March). Results: We found evidence of a U-shaped association between outdoor air temperature and spontaneous abortion risk based on year-round data. When restricting to warm season events (n=657), the OR for a 10-percentile increase in the mean of lag 0-6 daily maximum temperatures was 1.1 (95% CI: 0.96, 1.2) and, for the maximum, 1.1 (95% CI: 0.99, 1.2). The OR associated with any extreme heat days (>95th county-specific percentile) in the preceding week was 1.2 (95% CI: 0.95, 1.5). Among cool season events (n=615), there was no appreciable association between lower temperatures and spontaneous abortion risk. Conclusion: Our study provides evidence of an association between high outdoor temperatures and incidence of spontaneous abortion.