Aims We aimed to investigate associations between pre-pregnancy obesity, gestational diabetes (GDM), offspring body composition, and left ventricular diastolic and systolic function in early childhood. Methods This is an observational study, including 201 mother–child pairs originating from the Finnish Gestational Diabetes Prevention Study (RADIEL; 96 with GDM, 128 with pre-pregnancy obesity) with follow-up from gestation to 6-year postpartum. Follow-up included dyads anthropometrics, body composition, blood pressure, and child left ventricular function with comprehensive echocardiography (conventional and strain imaging). Results Offspring left ventricular diastolic and systolic function was not associated with gestational glucose concentrations, GDM, or pregravida obesity. Child body fat percentage correlated with maternal pre-pregnancy BMI in the setting of maternal obesity ( r = 0.23, P = 0.009). After adjusting for child lean body mass, age, sex, systolic BP, resting HR, maternal lean body mass, pre-gestational BMI, and GDM status, child left atrial volume increased by 0.3 ml (95% CI 0.1, 0.5) for each 1% increase in child body fat percentage. Conclusions No evidence of foetal cardiac programming related to GDM or maternal pre-pregnancy obesity was observed in early childhood. Maternal pre-pregnancy obesity is associated with early weight gain. Child adiposity in early childhood is independently associated with increased left atrial volume, but its implications for long-term left ventricle diastolic function and cardiovascular health remain unknown. Electronic supplementary material The online version of this article (10.1007/s00592-020-01571-z) contains supplementary material, which is available to authorized users.