Identifying critical periods of susceptibility for maternal exposure to biothermal stress and the risks of stillbirth and spontaneous preterm birth in Western Australia.

A few studies investigated critical periods of temperature and the risks of stillbirth and preterm birth. This study aimed to identify critical periods of composite biothermal stress (Universal Thermal Climate Index, UTCI) for stillbirth and spontaneous preterm birth (sPTB). From the Midwives Notification System, 415,271 singleton births between 1st January 2000 and 31st December 2015 were linked to spatiotemporal UTCI in Western Australia. Covariate-adjusted weekly and monthly distributed lag non-linear Cox regression from twelve weeks before conception to birth were performed. Relative to median exposure (14.2 °C), extreme UTCI levels (1st-10th and 90th-99th centiles) were associated with higher hazards of stillbirth and sPTB, especially stronger at lower than higher exposures. Critical susceptible periods at 1st centile (10.2°C) exposure were found during gestational weeks 21-42 with the strongest hazard of 1.14 (95% CI 1.03, 1.27) in the 42nd week for stillbirth and during gestational weeks 26-36 with the strongest hazard of 1.09 (95% CI 1.06, 1.12) in the 36th week for sPTB. Monthly exposure showed a similar pattern but with greater magnitude. Mid to late gestation showed critical susceptible periods of biothermal stress on the birth outcomes, suggesting further studies and timely climate-related healthcare interventions.