Objective: To determine whether cardiac functional and structural changes in fetuses of mothers with gestational diabetes mellitus (GDM) persist in the offspring beyond the neonatal period. Design: Longitudinal study Setting: Fetal Medicine Unit in a UK teaching hospital Population: 73 women with GDM and 73 women with uncomplicated pregnancy were recruited and fetal cardiac scans were performed at 35-36 weeks’ gestation. Repeat echocardiogram was performed in their offspring during infancy. Main outcome measures: Fetal and infant cardiac functional and structural changes Results: Fetuses of mothers with GDM, compared to controls, had more globular right ventricles (sphericity index 0.7, IQR 0.6/0.7 vs 0.6, IQR 0.5/ 0.6, p<0.001) and reduced right global longitudinal systolic strain (-16.4, IQR -18.9/-15.3 vs -18.5, IQR -20.6/-16.8, p=0.001) and left global longitudinal systolic strain (-20.1, IQR -22.5/-16.9 vs -21.3, IQR -23.5/-19.5), p=0.021). In the GDM group, compared to controls, in infancy there was higher left ventricular E/e’ (8.7, IQR 7.3/9.7 vs 7.9 IQR, 6.8/8.9 p=0.011) and lower left ventricular global longitudinal systolic strain (-21.0, IQR -22.5/-19.4 vs -22.3, IQR -23.5/-20.7, p=0.001) and tricuspid annular plane systolic excursion (13.8, IQR 12.7/16.1 vs 15.2, IQR 13.8/16.8, p=0.003). These differences remained following multivariable analysis. Conclusion: GDM is associated with alterations in fetal cardiac function and structure compared to controls and persistent cardiac changes in infancy.