Associations between ambient temperature and risk of preterm birth in Sweden: a comparison of analytical approaches

BACKGROUND AND AIM: Evidence indicates that high temperatures are a risk factor for preterm birth. However, the large heterogeneity of study designs and statistical methods across previous studies complicate comparisons. We investigated associations of short-term exposure to ambient temperature with preterm birth in Sweden, applying three complementary analytical approaches. METHODS: We included 560,615 singleton live births between 2014-2019, identified in the Swedish Pregnancy Register. We estimated at residential address weekly mean temperatures at 1-km² spatial resolution using a spatiotemporal model. The main outcomes of the study were gestational age in weeks (GW) and subcategories of preterm birth (<37GW): extremely preterm birth (<28GW), very preterm birth (from 28 to <32GW), and moderately preterm birth (from 32 to<37GW). Case-crossover, quantile regression and time-to-event analyses were applied to estimate the effects of short-term exposure to ambient temperature during the week before birth on preterm births. Furthermore, distributed lag nonlinear models (DLNM) were applied to identify susceptibility windows of exposures throughout pregnancy in relation to preterm birth. RESULTS: A total of 1,924 births were extremely preterm (0.4%), 2,636 very preterm (0.5%), and 23,664 moderately preterm (4.2%). Consistent across all three analytical approaches (case-crossover, quantile regression and time-to-event analyses), higher ambient temperature (95th vs 50th percentile) demonstrated increased risk of extremely preterm birth, but associations did not reach statistical significance. In DLNM models, we observed no evidence to suggest an increased effect of high temperature on preterm birth risk. Even so, a suggested trend was observed of a higher risk of extremely preterm birth with higher temperature during the last week before birth. CONCLUSIONS: In Sweden, with high quality data on exposure and outcome, we did not find an association between high ambient temperatures and preterm births. Results were consistent across three complementary analytical approaches. KEYWORDS: Preterm births; ambient temperature; case-crossover; quantile regression; time-to-event analyses