Diastematomyelia is a rare malformation characterized by complete or incomplete division of the spinal cord by osseous or fibrocartilaginous septum. Most cases are seen in association with other anomalies of the vertebral column such as spina bifida, kyphoscoliosis, butterfly vertebra, and hemivertebra. In this report we describe two cases of isolated diastematomyelia detected at routine second-trimester detailed ultrasound scan, the most striking feature being the detection of an echogenic focus in the posterior aspect of the spine in association with widening of the interpedicular vertebral space. The prenatal literature is reviewed to assess the clinical significance of this finding.