Maternal exposure to high ambient temperature and risk of stillbirth in South Australia: a statewide cohort study, 2000–2021

Recently, pregnant women have been identified as susceptible to the effects of high ambient temperature. Evidence suggests that high temperature increases the risk of stillbirth, however, how this association differs by social disadvantage remains unclear. The aim of this study is to investigate the effect of maternal exposure to high ambient temperature on the risk of stillbirth, overall, and by social disadvantage. Maternal and birth information from 1 January 2000 to 31 December 2021 were obtained from the South Australian Perinatal Statistics Collection. We implemented a cohort study design of 417,207 births including 2,776 (0.7%) stillbirths. Mixed-effect logistic regression models with odds ratios (OR) and 95% confidence intervals (95% CI) were used to examine the effect of maternal exposure to daily maximum temperature at birth (lag day 0), and for each individual day in the week preceding delivery (lag days 1-7). Adjustments for low, medium or high disadvantage were considered in our models. Maternal exposure to high temperature ≥ 95th percentile (36.1 °C) in the week before delivery slightly increased the risk of stillbirth on lag day 2 (aOR: 1.10, 95%CI: 0.93-1.30, p = 0.26) and slightly decreased risk on lag day 3 (aOR: 0.90, 95%CI: 0.75-1.08, p = 0.26); however, the confidence intervals were wide and included 1, consistent with a null effect. Social disadvantage made negligible difference to the estimated odds of stillbirth associated with maximum temperature ≥ 95th percentile in the week before delivery. We recommend pregnant women consider adaptation strategies during periods of high temperature to minimise the risk of stillbirth.