Résumé : Introduction Our aim was to evaluate the intra- and inter-observer variability and the impact of operator experience on the estimation of fetal weight (EFW) as measured by 2-dimensional ultrasound (2D-US) and magnetic resonance (MR) imaging. Material and methods We estimated fetal weight in 46 singleton pregnancies at 35.6–41.4 weeks gestation using 2D-US according to the Hadlock formula and using MR imaging according to the equation developed by Baker. Each examination was performed twice, once by an inexperienced operator and once by an experienced operator. The MR-EFW was derived from the planimetric measurement of fetal body volume (FBV) using an assisted semi-automated method. Intra- and inter-observer variability was evaluated by Bland-Altman analysis. Regression analysis was used to investigate the effect of maternal BMI, delivery weight, diabetes and fetal gender on the differences in US-EFW between the inexperienced and experienced operators. Results US-EFW showed higher intra-observer variability than MR-EFW, irrespective of operator experience. The 95% limits of agreement of MR were narrower compared with those of the US measurements. Similarly, US-EFW showed higher inter-observer variability than MR-EFW. MR-EFW improvement over 2D-US for the limits of agreement was 77.9% for intra-observer variability and 74.5% for inter-observer variability. Regression analysis showed that the differences between US-EFW measurements were not related to any of the tested variables. Conclusions Operator experience has a marginal impact on the variability of US-EFW and no impact on MR-EFW variability. The variability in US-EFW measurements is unpredictable.