·         Alobar holoprosencephaly- This condition is produced when the two cerebral hemispheres fail to divide. Alobar type is the most severe form with a monoventricle, a dorsal sac of varying size and fused thalami. The falx, corpus callosum, optic and olfactory tracts are absent. The midbrain, brainstem and cerebellum usually appear normal. The presence of anterior cerebral cortex and hippocampal ridge differentiates this condition from hydranencephaly. The head in the alobar type is usually small. Identification of cavum septum pellucidum excludes the diagnosis of holoprosencephaly of all types. Associated anomalies, particular of the face, are common.

·         Hydranencephaly- Hydranencephaly is seen as a large monoventricular cystic structure within the fetal cranium with no cerebral cortex. The falx is usually present but the thalami absent. The condition represents a destructive brain process, most probably as a result of an in utero vascular accident. The choroid plexus may be preserved so that the head may be small, but with a functioning choroid plexus hydrocephalus may develop. There are normally no associated anomalies but the prognosis is poor.

·         Aqueduct stenosis- This is related to narrowing of the aqueduct which may be secondary to inflammation, maldevelopment or rarely inherited in X-linked manner. There is enlargement of both lateral and third ventricle. The fourth ventricle and cerebeilum are usually normal.

·         Spinal dysraphism- This refers to abnormal or incomplete fusion of the midline dorsal region during embryogenesis. It may be divided into two groups: the open form includes an intracanalicular lipoma, diastematomyelia (presenting as a central bright linear echo within the spinal canal of a widened level) and a tethered cord. Ventriculomegaly is often associated.

·         Meckel's syndrome- This is an autosomal recessive disorder associated with a posterior encephalocele, hydrocephalus, small head diameter, large kidneys due to bilateral non-obstructive dysplasia, empty bladder and OH.

·         Choroid plexus papilloma- These are small benign tumors which arise from the normal choroid plexus. They are echogenic masses and can be differentiated from blood clot by their pulsating vascularity. They may cause hydrocephalus; sonography may reveal a mass within a dilated ventricle.

·         Cloverleaf skull deformity with hydrocephalus- The term Kleeblattschãdel means cloverleaf. The cloverleaf skull is an enlarged trilobed head deformity which results from premature fusion of the coronal and lambdoid sutures, associated with hydrocephalus. The anomaly is subdivided into type I (the more severe form associated with thanatophoric dysplasia) and type II (associated with less severe skeletal abnormalities). Type III is an isolated anomaly with the best prognosis. Differential diagnosis is from craniostenosis, ABS and Apert’s syndrome (congenital craniostenosis and syndactyly of hands and feet, associated visceral malformations include dextrocardia and pyloric stenosis.)

·         Arachnoid cysts- most arachnoid cysts are acquired. Congenital cysts are extremely rare . these cysts produce a mass effect within the fetal cranium and may result in hydrocephalus. Arachnoid cysts must be distinguished from other supratentorial cysts.

·         Intracranial bleeds- theses occure in the third trimester and result in secondary obstruction and at the aqueduct level.

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