This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
Hydrocephalus is best detected antenately by: 
Select and explain. 
A. X-ray abdomen, B. Amniocentesis, C. Clinical examination, D. Ultrasonography
D: Ultrasonography
Answer: D: Ultrasonography 
Explanation: Hydrocephalus is a condition in which there is an abnormal increase in cerebrospinal fluid within the ventricular and subarachnoid spaces of brain.
“The prenatal diagnosis of hydrocephalus is usually made by demonstration of a dilated ventricular system in an ultrasound examination.”
Fernando Arias 2/e, p 331

Earliest and most accurate sonographic sign of hydrocephalus – Enlarged lateral ventricles.
The lateral ventricle is measured at the level of atrium.
Normal transverse diameter of atrium is 7 mm + 1 mm (It remains constant during the second and third trimester).
When diameter of atrium is >10 mm, it is called as Ventriculomegaly/Hydrocephalus.

Other signs of hydrocephalus on USG:

Dangling choroid plexuses.
Thinning out of cerebral cortex.