Adverse birth outcomes in Victoria, Australia in association with maternal exposure to low levels of ambient air pollution

Background: The relationship between maternal exposure to air pollution and birth outcomes is not well characterised where ambient air pollution is relatively low. Objectives: We aimed to explore the association between maternal exposure to ambient nitrogen dioxide (NO2) and fine particulate matter (PM2.5) and a range of birth outcomes in Victoria, Australia. Secondary aims were to explore whether obstetric conditions, such as gestational diabetes mellitus (GDM) and hypertensive disorders of pregnancy, were effect modifiers in observed relationships. Methods: We included all singleton births occurring in Victoria, Australia from 1st March 2012 to 31st December 2015 using routinely collected government data. Outcomes included birth weight, small for gestational age (SGA), term low birth weight (tLBW), large for gestational age (LGA), and spontaneous preterm birth (sPTB). We estimated exposure to annual ambient NO2 and PM2.5 concentrations, assigned to maternal residence at time of birth. Confounders included maternal, meteorological and temporal variables. Multivariable linear regression and log-binomial regression were used for continuous and dichotomous outcomes, respectively. Results: There were 285,594 births during the study period. Average NO2 exposure was 6.0 parts per billion (ppb, median 5.6; interquartile range (IQR) 3.9) and PM2.5 was 6.9 mu g/m(3) (median 7.1, IQR 1.3). IQR increases in ambient NO2 and PM2.5 were associated with fetal growth restriction, including decrements in birth weight (NO2 beta - 22.8 g; 95%CI -26.0, -19.7; PM2.5 beta - 14.8 g; 95%CI -17.4, -12.2) and increased risk of SGA (NO2 RR 1.08; 95%CI 1.06, 1.10; PM2.5 RR 1.05; 95%CI 1.04, 1.07) and tLBW (NO2 RR 1.06; 95%CI 1.01, 1.10; PM2.5 RR 1.04; 95%CI 1.03, 1.08). Women with GDM and hypertensive disorders of pregnancy had greater decrements in birth weight in association with pollutant exposure. Discussion: In this exploratory study using an annual metric of exposure, maternal exposure to low-level ambient air pollution was associated with fetal growth restriction, which carries substantial public health implications.