Objectives To examine the relationship between prenatal nitrogen dioxide (NO₂) exposure and foetal growth in a prospective cohort of 1001 Chinese women.

Methods The maternal NO₂ exposure levels were estimated using land-use regression models based on home address. The biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL) and estimated foetal weight (EFW) were evaluated via ultrasonography. The multiple linear regression model was used to adjust for confounders, and the mixed-effect model was used to assess longitudinal effect.

Results With a 10 μg/m³ increase in NO₂ exposure, the BPD, HC, AC, FL and EFW in the second trimester decreased by 0.40 mm (95% CI −0.56 to −0.24), 1.07 mm (95% CI −1.60 to −0.54), 1.02 mm (95% CI −1.57 to −0.48), 0.24 mm (95% CI −0.37 to −0.12) and 7.84 g (95% CI −11.59 to −4.08), respectively; the BPD and HC in the third trimester decreased by 0.26 mm (95% CI −0.50 to −0.02) and 0.71 mm (95% CI −1.37 to −0.06), respectively. The longitudinal analyses showed inverse associations of NO₂ exposure with BPD, HC, AC and FL (all p<0.05). The stratified analyses showed that the effects of NO₂ on the HC, FL and EFW in the second trimester were stronger among female babies and that the effect of NO₂ on EFW in the third trimester was stronger among smoking mothers (all p<0.05).

Conclusions In this prospective study of Chinese women, maternal NO₂ exposure was inversely associated with foetal growth, and the association was stronger among female babies and smoking mothers.