MTHFD1 and ventricular septal defect: Based on the abovementioned background, we performed a hospital-based, case-control study in Han Chinese populations to (1) assess the association between maternal dietary habits in the first-trimester pregnancy and the risk of VSD, (2) determine the relationship between the maternal MTHFD1 gene polymorphisms and the risk of VSD, and 3) investigate the interaction effect of maternal MTHFD1 gene polymorphisms and dietary habits on the risk of VSD.