OBJECTIVE: To study the prevalence of abnormalities of the septi pellucidi (SP) in a cohort of fetuses with open spinal dysraphism (OSD) and to determine whether this condition is secondary to obstructive ventriculomegaly and, therefore, part of the natural history of prenatal intracranial hypotension (PICH) syndrome. METHODS: Magnetic resonance imaging (MRI) studies from fetuses with OSD were analyzed. The SP were assessed using axial and coronal T2-weighted images of the fetal brain and classified as intact, partially absent, or completely absent. Additionally, the correlation between the presence or absence of the SP and the size of the lateral ventricles, degree of cerebellar tonsillar herniation, collapse of the fourth ventricle, and interpeduncular angle was investigated. RESULTS: A total of 32 fetuses with OSD were studied. Mean gestational age at the time of the fetal MRI was 25.5 ± 3.9 weeks (range, 19-35) and mean ventricular size was 16.2 ± 4.2 mm (range, 8-26). Twenty-three (71.9%) fetuses had cerebellar tonsillar herniation. The IPA was completely collapsed in 23 cases (71.9%), reduced in seven (21.9%), and unreadable in two (6.3%). Twenty (62.5%) fetuses presented with intact SP, 10 (31.3%) with partially absent SP (incomplete fenestration), and two (6.3%) with completely absent SP (complete fenestration). Fenestration of the SP correlated significantly with the degree of ventriculomegaly (Pearson's correlation coefficient =0.459; p = .01). However, there was no correlation with the IPA, collapse of the fourth ventricle, and cerebellar tonsillar herniation. CONCLUSIONS: More than one-third of the fetuses with OSD had fenestration of the SP. The most probable etiology is increased intraventricular pressure leading to local necrosis of the SP. As fenestration of the SP is a secondary event associated with PICH syndrome, this condition should not be considered a contraindication for intrauterine repair of the spinal defect. Instead, it should be seen as an indicator of the severity of the intraventricular pressure.