Ambient temperature and air quality in relation to small for gestational age and term low birthweight

Background: Exposures to extreme ambient temperature and air pollution are linked to adverse birth outcomes, but the associations with small for gestational age (SGA) and term low birthweight (tLBW) are unclear. We aimed to investigate exposures to site-specific temperature extremes and selected criteria air pollutants in relation to SGA and tLBW. Methods: We linked medical records of 220,572 singleton births (2002-2008) from 12 US sites to local temperature estimated by the Weather Research and Forecasting model, and air pollution estimated by modified Community Multiscale Air Quality models. Exposures to hot (> 95th percentile) and cold (< 5th percentile) were defined using site-specific distributions of daily temperature over three-month preconception, each trimester, and whole -pregnancy. Average concentrations of five criteria air pollutants and six fine particulate matter constituents were also calculated for these pregnancy windows. Poisson regression with generalized estimating equations calculated the relative risks (RR) and 95% confidence intervals for SGA (weight < 10th percentile conditional on gestational age and sex) and tLBW (>= 37 weeks and < 2500 g) associated with an interquartile range increment of air pollutants, and cold or hot compared to mild (5-95th percentile) temperature. Models were adjusted for maternal demographics, lifestyle, and clinical factors, season, and site. Results: Compared to mild temperature, cold exposure during trimester 2 [RR: 1.21 (1.05-1.38)], trimester 3 [RR: 1.18 (1.03-1.36)], and whole-pregnancy [RR: 2.57 (2.27-2.91)]; and hot exposure during trimester 3 [RR: 1.31 (1.15-1.50)] and whole -pregnancy [RR: 2.49 (2.20-2.83)] increased tLBW risk. No consistent association was observed between temperature and SGA. Air pollutant analyses were generally null but preconception elemental carbon was associated with a 4% increase in SGA while dust particles increased tLBW by 10%. Particulate matter 510 gm in the second trimester and whole pregnancy also appeared related to tLBW. Conclusions: Our findings suggest prenatal exposures to extreme ambient temperature relative to usual environment may increase tLBW risk. Given concerns related to climate change, these findings merit further investigation.