Maternal exposure to fine particulate matter and congenital heart defects during preconception and pregnancy period: A cohort-based case-control study in the Taiwan maternal and child health database

•Few studies investigated the effect of prenatal and gestational PM2.5 exposure on the CHDs.•PM2.5 was associated with CHDs during weeks 7–12 before conception.•PM2.5 was associated with CHDs during weeks 3–9 after conception.•Low socioeconomic status is more sensitive to the adverse impact of PM2.5 on CHDs.