The antenatal diagnosis of coarctation of the aorta can be life saving; however, it is notoriously difficult to predict using ultrasound alone. We present 16 fetal cases with suspected coarctation of the aorta between 30-36 weeks, assessed using a combination of novel prenatal MRI techniques: three-dimensional motion corrected slice-volume registration and metric-optimised gated phase contrast flow measurements. Detailed 3D analysis revealed significant correlation between the size and position of the aortic isthmus and the need for neonatal surgical repair. Retrograde flow at the aortic isthmus and the angle of its insertion relative to the arterial duct may also be important variables. The comprehensive physiological data generated by combining novel fetal MRI techniques may offer powerful insights into traditionally challenging antenatal diagnoses.