Objectives: Blood vessel resistance is assessed by flow velocity indexes. However, the actual resistance should be more strongly correlated with vessel diameter rather than variations in flow velocity. We aimed to determine the utility of middle cerebral artery (MCA) diameter measurement as a tool for evaluating brain sparing effect in intrauterine growth-restricted (IUGR) discordant twins. Methods: Ten dichorionic-diamniotic twin pairs with weight discordance were included in the study. Fetal assessment included estimated fetal weight, umbilical artery flow, MCA flow and MCA diameter. Paired statistical analysis was used to compare MCA diameter and MCA pulsatility index (PI) between the discordant twins. Results: MCA diameter was significantly larger in the smaller twin (mean diameter 3.55±0.26 vs 2.71±0.22, P=0.018, mean ratio 1.39±0.14). There was no significant difference in MCA PI values within the twin pairs (mean PI 1.51±0.13 vs 1.57±0.07, P=0.878, mean ratio 0.99±0.11). Conclusions: Measurement of MCA diameter can potentially be used as a tool for assessing vessel resistance. Further studies are needed to assess the feasibility and sensitivity of this method as well as the clinical significance of MCA dilatation in the presence of normal Doppler flow indexes.