Abstract
BackgroundNumerous studies have examined the association between air pollution and preterm birth (< 37 weeks gestation) but findings have been inconsistent. These associations may be more difficult to detect than associations with other adverse birth outcomes because of the different duration of exposure in preterm vs. term births, and the existence of seasonal cycles in incidence of preterm birth.MethodsWe analyzed data pertaining to 1,001,700 singleton births occurring between 1999 and 2008 in 24 Canadian cities where daily air pollution data were available from government monitoring sites. In the first stage, data were analyzed in each city employing Cox proportional hazards models using gestational age in days as the time scale, obtaining city-specific hazard ratios (HRs) with their 95% confidence intervals (CIs) expressed per interquartile range (IQR) of each air pollutant. Effects were examined using distributed lag functions for lags of 0–6 days prior to delivery, as well as cumulative lags from two to six days. We accounted for the potential nonlinear effect of daily mean ambient temperature using a cubic B-spline with three internal knots. In the second stage, we pooled the estimated city-specific hazard ratios using a random effects model.ResultsPooled estimates across 24 cities indicated that an IQR increase in ozone (O3, 13.3 ppb) 0–3 days prior to delivery was associated with a hazard ratio of 1.036 (95% CI 1.005, 1.067) for preterm birth, adjusting for infant sex, maternal age, marital status and country of birth, neighbourhood socioeconomic status (SES) and visible minority, temperature, year and season of birth, and a natural spline function of day of year. There was some evidence of effect modification by gestational age and season. Associations with carbon monoxide, nitrogen dioxide, particulate matter, and sulphur dioxide were inconsistent.ConclusionsWe observed associations between daily O3 in the week before delivery and preterm birth in an analysis of approximately 1 million births in 24 Canadian cities between 1999 and 2008. Our analysis is one of a limited number which have examined these short term associations employing Cox proportional hazards models to account for the different exposure durations of preterm vs. term births.
Highlights
Preterm birth is a key determinant of infant mortality and morbidity, and of health status in childhood and even adulthood [1,2,3]
After exclusion of births with missing covariate data, 1,001,700 births were included in the analysis including 63,400 preterm births, resulting in an overall prevalence of preterm birth of 6.34%
Our findings for PM2.5 and Nitrogen dioxide (NO2) were similar to our earlier analysis where we found null or negative associations of preterm birth with PM2.5 or NO2 averaged over gestational month, trimester or the entire pregnancy [12, 13]
Summary
Preterm birth is a key determinant of infant mortality and morbidity, and of health status in childhood and even adulthood [1,2,3]. It has been hypothesized that the association between air pollution and preterm birth may be more difficult to detect than associations with other outcomes such as term low birth weight or small for gestational age because of the different duration of exposure over the entire pregnancy or third trimester in preterm vs term births, and the existence of seasonal cycles in incidence of preterm birth [15, 21, 27, 28] To address these issues and to examine the influence of short-term exposure, here we employ a time to event analysis, using Cox models examining exposures in the week prior to birth. Numerous studies have examined the association between air pollution and preterm birth (< 37 weeks gestation) but findings have been inconsistent These associations may be more difficult to detect than associations with other adverse birth outcomes because of the different duration of exposure in preterm vs term births, and the existence of seasonal cycles in incidence of preterm birth
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