Introduction Brainstem disconnection syndrome is a rare and severe disease resulting from a midbrain–hindbrain segmental defect. Clinical signs include a severe neurological impairment, an early death (usually during the first year of life), and pathognomonic postnatal brain imaging features. Two major hypotheses are proposed to explain the etiopathogenesis of this syndrome, namely an inborn error of morphogenesis or a vascular disruption defect. Case Report and Literature Review Here we report on prenatal (ultrasound; fetal MRI) and postnatal (MRI) neuroimaging findings observed in a full‐term female newborn with a brainstem disconnection syndrome. The prenatal and postnatal findings point toward an early fetal vascular disruption defect as the pregnancy was marked by three episodes of hospitalization resulting from a very severe maternal dehydration. The first episode took place as early as the 18th week of gestation. Our clinical follow‐up at 1 year age is well in line with the findings observed in 13 other cases reported in the literature. Interestingly, among these 13 cases, a vascular disruption defect was suggested in 8 patients and confirmed by autopsy in at least 2 cases. Conclusion In the present report, we bring objective evidence for the antenatal cause of a brainstem disconnection syndrome resulting from a vascular disruption defect occurring in the context of a severe maternal dehydration. In particular, our neuroimaging findings observed during pregnancy and after birth illustrate the prenatal occurrence of this vascular disruption defect.