A meningocele represents protrusion of the meninges through a vertebral defect, with the spinal cord resting in its normal position within the spinal canal. A myelomeningocele consists of protrusion of the meninges which contain spinal cord and nerve roots. These malformations result from failure of the caudal neural tube to close at 3 to 4 weeks. Myelomeningocele is a more common and severe defect than meningocele and has an incidence of 1 in 1,000 births.

Sonographically, myelomeningocele can be diagnosed when the posterior ossification centers of the spine are splayed outward on a transverse image and are farther apart than the ossification centers above and below the defect. A cleft in the adjacent soft tissues is also usually present. The myelomeningocele sac is easily recognized when it bulges into the amniotic cavity, as a cystic or soft tissue extension of the posterior aspect of the spine. However, if the sac abuts the myometrium or placenta or is flattened or intact, the abnormal morphology of the posterior ossification centers of the spine must be used to make the diagnosis.

Sonographic findings associated with myelomeningocele include lateral cerebral ventricular dilatation (seen in 80 to 90% of cases but not seen in this case), effacement of the cisterna magna, the "banana sign," and the "lemon sign" (Film 4). Ventriculomegaly is defined as a ventricular atrium which measures >10 mm. It is most often associated with the Arnold-Chiari malformation. The abnormalities of the posterior fossa are also attributed to the Arnold-Chiari malformation. Effacement of the cisterna magna is thought to be secondary to posterior fossa compression, with the "banana sign" representing the cerebellum wrapped around the posterior brain stem secondary to downward traction of the spinal cord. The "lemon sign" consists of a concave or linear frontal contour of the fetal calvarium, the etiology of which is uncertain. It is possible that low intraspinal pressure, which has been observed in the majority of neonates with myelomeningoceles, might be translated to the cranium and result in the inward deformity.