In this prospective study, we aimed to establish the value of volumetric assessment by prenatal brain MRI in determining the prognosis of fetuses with isolated VM. A total of 23 fetuses with isolated VM were included in the study. Supratentorial cerebral parenchyma volume (PV) and ventricular volume (VV) were measured, and supratentorial ventricular/parenchymal volume (VV/PV) ratios were calculated. Pregnancy and postnatal neurodevelopmental outcomes up to two years of age were obtained and correlated with the volumetric measurements. VV was found to be strongly and positively correlated with ventricular dimension. There was a statistically significant difference between the VV/ PV ratios of the good and poor prognosis groups into which the cases had been categorized. The fetuses with a poor prognosis had a significantly higher VV/PV ratio. Volumetric parenchymal and ventricular measurements obtained by fetal brain MRI may contribute to future clinical studies concerning the evaluation of fetuses with VM and provide an important indicator in cases where management dilemmas arise.