Resumen: Objective: To report the long-term outcome of a series of fetuses with isolated septum pellucidum agenesis (ASP) with measurement of their optic chiasm by fetal neurosonography (NSG). Method: All patients with ASP and NSG evaluated from 2008 to date and with follow-up until their school age were included. In all cases, clinical, NSG and magnetic resonance imaging (MRI) data were recorded. Parents were interviewed by telephone. Results: Nine patients met the criteria: four with septo-optic dysplasia (SOD) (follow-up range: 5-14 years) and five without SOD (follow-up range: 7-10 years). A tenth case was excluded because only 6 months of follow-up. None of the ASP cases had another anomaly detected in their follow-up. None of the cases with DSO had anomaly of the size of their optic chiasm on NSG or abnormality in the optical pathway in the MRI. Conclusions: In our population, the residual risk of DSO versus ASP is 44.4%. At follow-up, our NSG definition of ASP had no false negatives in relation to other postnatal-onset anomalies, except for SOD.