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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

Introduction: Although the prevalence of preterm births ranges from 5 to 13% and represents the leading cause of perinatal mortality and morbidity in developed countries, the etiology of preterm birth remains uncertain. We aimed to evaluate the effect of short-term exposure to high and low temperatures and air pollution on preterm delivery and to identify socio-demographic and clinical maternal risk factors enhancing individual susceptibility. Methods: We analyzed all singleton live births by natural delivery that occurred in Rome in 2001–2010. A time-series approach was used to estimate the effect of exposure to minimum temperature, maximum apparent temperature, heat waves, particulate matter with an aerodynamic diameter of 10 μm or less (PM10), ozone, and nitrogen dioxide in the month preceding delivery; the analysis was conducted separately for cold and warm seasons. Socio-demographic and clinical risk factors were included as interaction terms. Results: Preterm births comprised nearly 6% of our cohort. An increase of 1.9% (95% confidence interval (CI) 0.86–2.87) in daily preterm births per 1 °C increase in maximum apparent temperature in the 2 days preceding delivery was estimated for the warm season. Older women, women with higher education levels, and women with obstetric or chronic pathologies reported during delivery had a lower effect of temperature on the risk of preterm birth, while women with a chronic disease in the two years before delivery and mothers