This was a 30 year old primi gravida with no history of consanguinity . This was referred for evaluation of suspected ventriculomegaly.

The findings were :

prominent ventriculomegaly of posterior horns of both lateral ventricles – colpocephaly

dilated occipital horns -colpocephaly

cavum septum pellucidum is not made out clearly

demonstration of both medial and lateral ventricular walls at a level where the single periventricular line is normally demonstrated

The following links would be very helpful to understand more :

http://www.sonoworld.com/fetus/page.aspx?id=107

http://www.fetalultrasound.com/online/text/5-077.HTM

A specific diagnosis of agenesis of the corpus callosum has seldom been made before the third trimester, probably because the corpus callosum is not normally formed until 18 to 20 weeks⁴³. Most authors agree that detection of agenesis of the corpus callosum is difficult prenatally, depending as it does on postnatal sonograms or CT or MR scans. In a series of seven fetuses with agenesis of the corpus callosum, Bertino et al³⁵ reported that only three demonstrated a characteristic midline cyst. They described three findings that might lead one to suspect agenesis of the corpus callosum on routine transverse views:

• disproportionate enlargement of the occipital horn,

• demonstration of both medial and lateral ventricular walls at a level where the single periventricular line is normally demonstrated, and

• a more parallel course of both ventricular walls than normal.

They suggested that demonstration of these findings on axial views should stimulate additional coronal and sagittal views for evaluation of agenesis of the corpus callosum.

The increased separation of the normal-sized bodies and the enlargement of the atria and occipital horns of the lateral ventricle result in a typical ultrasound image. Upward displacement of the third ventricle is a very specific sign³⁶ but presents only in 40% of fetuses.