Limitations
We acknowledge several limitations. First, its retrospective design may result in selection bias as we included only cases with more than one good quality MRI examination. Second, as a fetal surgery center, the group of fetuses with severe hypoplasia is overrepresented; in addition most of them received a prenatal intervention, which changes the natural history 6, 7, 24. Third, we have no standardized postnatal follow up information on these cases, as many patients do not deliver in our center.