This fetus was scanned at 16 weeks, and a cervical mass was observed. Note the spine on the first image: it is remarkable for the normal appearance of the posterior arch and the small posterior cystic lesion

The posterior fossa is normal, with no evidence of Arnold-Chiari malformation.

3D pictures clearly demonstrate that the mass was attached to the fetus. 

A coronal 3D reconstruction of the cervical spine is also provided. 

There is also a short (200K) videoclip.

No associated anomalies were identified at this time. The cerebral anatomy, including the infratentorial structures was considered normal, without an Arnold-Chiari malformation. Nuchal translucency performed at 13 weeks, maternal serum AFP, amniotic fluid AFP and fetal karyotype were normal. 

Findings:

The findings are limited to:

• a small cystic lesion
• posterior to the cervical spine
• without alteration of the posterior neural arch
• without intracranial findings

Although many readers got the answer of meningocele correct very few noticed or suspected that there was only a filiform connection between the meningocele and the spinal canal. This form of meningocele with a filiform connection is very rare to my knowledge and I have only seen one before at the lumbar level.

Sagittal and axial views of a different fetus with a similar lumbar meningocele with a filiform connection.

These are the views of the fetus.

The X-ray of the neck is normal.

The probe is in the filiform connection