Heat and air pollution exposure as triggers of delivery: A survival analysis of population-based pregnancy cohorts in Rome and Barcelona

Introduction: Environmental exposures have been linked to length of gestation but the question as to during which weeks of gestation pregnancies are most susceptible still remains little explored.We estimated the effect of maximum apparent temperature and air pollution levels on risk of birth by week of gestation. Methods: We analyzed two cohorts of singleton live births in Rome (2001–2010) and Barcelona (2007–2012). Maximum apparent temperature (MAT), PM10, O3 and NO2 were analyzed in the warm period (1st April–31st October).Gestational week-specific hazard ratios of giving birth associated to a 1-unit increase in exposure were estimated fitting Cox regression models adjusted for seasonality, and demographic and clinical characteristics of the mother. Results: We observed 78,633 births (5.5% preterm) in Rome and 27,255 (4.5% preterm) in Barcelona. The highest hazard ratios for 1 °C increase in MAT were in the 22nd–26th weeks of gestation, 1.071, (95% CI 1.052–1.091) in Rome and 1.071 (95% CI 1.036–1.106) in Barcelona, and decreased to 1.032 (95% CI 1.026–1.038) and 1.033 (95% CI 1.020–1.045) at the 36th week of gestation, respectively. Similar associations and trends were observed for PM10 and NO2 after adjusting for MAT. O3 showed similar trends but weaker associations. Conclusions: We found, consistently in Rome and Barcelona, an increased risk of delivery for a unit increase in MAT, PM10, NO2 and O3, especially in the second half of the second trimester, thus effectively increasing the risk of preterm and particularly early preterm birth. Results may help to increase awareness of these risks among public-health regulators and clinicians, leading to better preventive strategies. Keywords: Preterm, Gestational length, High temperature, Air pollutants, Birth cohorts