Background and Aim The prenatal diagnosis of posterior fossa dysgenesis remains challenging.

Mega-cisterna magna (MCM) is applied to prominence of the retro-cerebellar CSF space and may not be associated with abnormalities. The clinical significance of this finding varies. Diagnostic modalities of the MCM are by ultrasound and or MRI.

Our aim is to:

• Determine the Incidance of isolated dilated MCM in our population.

• Compare the accuracy of fetal ultrasound with postnatal neuroimaging.

• Report the associated central nervous system anomalies.

Method We identified retrospectively all reported cases of isolated dilated cisterna magna from 1/January 2007 till 31/7/2011.

All medical records and imaging studies were reviewed.

Results Number of Deliveries over the 5-year study period was 25443 of which 4100 (16%) were un-booked. Dilated cistern magna was diagnosed in 26 (0.12%) fetal scans (15 male and 11 females). Post natal scans were done in 20/26 (77%) and not done in 6/26 (23%) of cases.

Of the 20 post natal scans done 12 (60%) were normal and 8 (40%) were abnormal. The following abnormalities were detected: 4 (50%) dandy walker complex, 2 (25%) corpus collasum dysgenesis and 2 (25%) cerebellar hypoplasia. Neuro-developmental assessment was reported as normal in 60% of the isolated mega cistern magna patients.

Conclusion The association of mega cisterna magna with major CNS anomalies is high in our population. Post natal neuroimaging confirmation of all abnormal fetal sonography is required. Long term neurocognative assessment and follow up is essential for this population.