Narrowing/occlusion at aqueduct of Sylvius (between 3rd and 4th ventricles), causing obstructive hydrocephalus

Moderate to severe ventricular dilatation (> 15 mm)

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  May be extreme with macrocephaly

Dangling choroid: Choroid does not fill ventricle

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  When severe, upside choroid may fall through dilated foramen of Monroe into dependent ventricle

Normally formed posterior fossa: Cerebellum may be compressed by supratentorial ventriculomegaly

Male fetuses with X-linked form have adducted thumbs

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  Thumbs curled into palm of hand

Holoprosencephaly: Single communicating ventricle, absent falx

Hydranencephaly: Parenchyma destroyed, no cortical mantle

Developmental delay in up to 90%, severe in X-linked form

Posterior fossa images of critical importance to rule out other causes of ventriculomegaly

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  Normal in aqueductal stenosis, although can be compressed if hydrocephalus is severe

  
  
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  Often abnormal with other malformations

Document gender and carefully image hands in males looking for adducted thumbs (X-linked hydrocephalus)

Carefully assess for remaining cortical mantle

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  Differentiates aqueductal stenosis from destructive lesions or other congenital malformations

  
  
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  Use endovaginal probe if head is cephalic

  
  
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  Doppler to look for flow in compressed parenchyma