Migrant population is more vulnerable to the effect of air pollution on preterm birth: Results from a birth cohort study in seven Chinese cities

Studies have reported that exposure to air pollution during pregnancy was associated with preterm birth (PTB). However, it remains unknown whether this association differs between local residents and migrants. This study aimed to differentiate the associations between maternal air pollution exposure and PTB between local residents and migrants. We established a retrospective birth cohort in seven Chinese cities in Pearl River Delta (PRD) region during 2015–2017. The mothers were included in the cohort at their first time of hospital visit for pregnancy, and the endpoint events were identified using the birth registry. The air pollution exposure was estimated based on the daily air pollution concentrations in the nearby air monitoring stations during different pregnancy periods. Cox proportional hazards models were utilized to estimate the associations between each air pollutant and PTB for different pregnancy periods. Our cohort included a total of 628,439 mother-and-live-birth pairs. Among them, 308,201 women were local residents, and 320,238 were migrants. We observed stronger effects of air pollutants among the migrants than the local residents. For the exposure during the entire pregnancy, the hazard ratio (HR) among the migrants and local residents were 1.56 (95% CI: 1.50, 1.63) and 0.98 (95% CI: 0.93, 1.02) for each 10 μg/m3 increase in PM2.5, 1.32 (95% CI: 1.27, 1.39) and 1.18 (95% CI: 1.12, 1.23) for each 10 ppb increase in O3, and 1.48 (95% CI: 1.40, 1.57) and 0.99 (95% CI: 0.93, 1.05) for each 10 μg/m3 increase in SO2, respectively. Similarly higher effects were observed among the migrants for the exposures in different trimesters of pregnancy. However, the effects of NO2 were comparable between the two groups. Our study suggests that maternal PM2.5, O3 and SO2 exposures might be important risk factors of preterm birth, particularly among the migrants. More specific protective and education measures should be considered for the migrant pregnant women.