Preterm birth during an extreme weather event in Québec, Canada: a "natural experiment".

To clarify the relationship between preterm birth (PTB) and extreme weather events, we evaluated PTB during a January 1998 ice storm that had led to a provincial emergency in the middle of winter in the province of Québec, Canada. Singleton live births for three periods (1993-1997, 1998, 1999-2003) were obtained (N = 855,320). PTB was defined as gestational age <37 completed weeks. Births in the Triangle of Darkness, the area most strongly affect by the storm, were geocoded. Multivariate logistic regression was used to calculate the likelihood of PTB for the Triangle relative to metropolitan Montréal, adjusting for maternal age, education, civil status, maternal birthplace, and previous deliveries. Associations for 1998 relative to other periods were evaluated. Short-term (January-February) and long-term (March-October) exposure periods were examined. The proportion PTB for 1998 January-February births in the Triangle (8.7%) was high compared with 1998 March-October births (6.0%) and with the corresponding proportions for 1993-1997 (6.2%) and 1999-2003 (6.9%). Covariate-adjusted odds of PTB for January-February 1998 were 27% higher for the Triangle relative to metropolitan Montréal, though precision was low. Furthermore, adjusted odds were 28% higher for 1998 relative to 1999-2003, despite increasing rates of PTB over time. Odds were not elevated over a long-term exposure period. This study suggests a weak association between PTB and exposure to extreme weather for the two months following an ice storm, but not for later periods after the storm.