Preterm birth and ambient temperature: Strong association during night-time and warm seasons.

BACKGROUND: Mounting evidence have linked high temperature with increased incidence of preterm birth (PTB), but which time period(s) of increase in temperature within a day and a year during pregnancy play an important role in PTB is not clarified.

OBJECTIVE: This study aims to determine the critical time period(s) of heat exposure on the development of PTB by exploring the risk of PTB for temperature increase during 24 h of a whole day and each of four seasons during pregnancy.

METHODS: A cohort study was performed with 3509 preschool children in Changsha, China, which evaluated the prevalence of PTB using a questionnaire filled out by the children's parents. The authors estimated the temperature at which each mother was exposed to outdoor air temperature during three trimesters and for 24 h in different seasons of pregnancy. The logistic regression models in regard to odds ratio (OR) and 95% confidence interval (CI) was applied to study associations of PTB with an increase in temperature during different time windows.

RESULTS: Prevalence of PTB (4.1%) was associated with outdoor temperature with adjusted ORs (95% CI) = 1.34 (1.11-1.62) for increase of 1 °C in temperature exposure during pregnancy, and this association was significantly higher in spring and summer than autumn and winter. Specifically, high temperature during summer night posed the highest risk of PTB, with peaks during 03:00-04:00 a.m. We further found that PTB was associated with exposure to temperature and extreme heat day during the 2nd trimester, with ORs = 1.03 (1.00-1.06) and 1.01 (1.00-1.02) respectively. Foetus with male sex and older mothers were more sensitive to the impact of heat exposure during pregnancy on PTB, with higher risk observed during night time.

CONCLUSION: Our study indicates that heat exposure was related to an elevated risk of PTB, with higher risk of PTB for nocturnal exposure and warm seasons. Our findings have implications for the adverse effect of elevated temperature in the future as global warming continues.