Background: Maternal exposures to air pollution in early pregnancy may affect subsequent fetal growth. Aims: We examined the association between first trimester maternal exposure to air pollution and ultrasound-assessed indicators of fetal growth during pregnancy. Methods: Data were collected during multiple prenatal visits of 274 mother-fetal pairs recruited in Mexico City, Mexico between 2009-2012. Individual-specific first trimester exposures to PM2.5 and 03 were estimated using citywide average concentrations reported from ambient air monitors. Random slope and intercept mixed effect models were fit to assess pollution associations with two ratio measures from repeated fetal ultrasounds: femur length (FL) to abdominal circumference (AC) (FL/AC); and head circumference (HC) to AC (HC/AC). Clinical indicators of normal (proportional) growth for HC/AC and FL/AC ratios are 1.1 and 0.20-0.22, respectively. Ratios above the 95th percentile may represent fetal growth restriction. Models were adjusted for maternal age, pre-pregnancy BMI, vitamin intake and stratified by fetal sex, as in utero growth differs by sex. Results: On average, the FL/AC ratio was 0.22 (±0.02) and the HC/AC ratio was 1.09 (±0.10). Women exposed to 03 levels higher than 16.05 ppb during the first trimester showed a decrease of 0.007(p=0.01) in FL/AC ratio during gestation; associations did not differ by fetal sex. However, change in FL/AC ratios per 10 μg/m3 (IQR) increments of PM2.5 were -0.0005 (p= 0.90) for males and 0.0050 (p=0.23) for females. HC/AC ratios increased with higher exposure to PM2.5 (0.0100, p=0.28) and 03 (0.0005,p=0.97); point estimates did not differ by fetal sex. Conclusions: First trimester exposures to air pollutants may affect indicators of in utero fetal growth uniquely; with responses differing by fetal sex. Further analyses will use participant's geo-referenced location for air pollution exposure analyses and evaluate effect modification by nutritional factors.
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