Potential mediators
To facilitate interpretation of our findings regarding the association between COVID lockdown and preterm birth, we considered pregnancy complications (HDP and GDM) and changes in air pollution around lockdown beyond regular seasonal variation as two potential mediators. HDP included gestational hypertension, preeclampsia/eclampsia, chronic hypertension, and chronic hypertension with superimposed preeclampsia (25,26) (Table S2). Daily air pollutant (PM10, PM2.5, NO2, SO2, and CO) data in the selected cities during 2015-2020 were collected from the National Urban Air Quality Real-time Publishing Platform (http://106.37.208.233:20035/). The average air pollutant concentrations during, after the Level I lockdown in 2020, and during the same calendar months in 2015-2019 were calculated.