Congenital hydrocephalus is sometimes diagnosed before birth with a fetal ultrasound. However, most cases are diagnosed during a physical exam at or soon after birth. Congenital hydrocephalus usually is first suspected because of a larger-than-normal head size. A doctor will also look for other physical signs, such as eyes that almost always look down and a lot of white showing above the irises ("sunsetting of the eyes"). A doctor will also ask about how your baby is eating and sleeping or whether he or she has seemed fussy.

A health professional may request one or more tests to confirm a diagnosis of congenital hydrocephalus or to further evaluate the condition. Imaging tests may be used to determine whether extra fluid is building in the brain, to look at the brain's structure, or to evaluate the flow of cerebrospinal fluid within and through the brain's ventricles and into the spinal column. These tests include:

• A cranial ultrasound, if the soft spots (fontanelles) are still open. Open fontanelles make it easier to see the brain during an ultrasound.
• A computed tomography (CT) scan of the head or a CT scan of the spine.
• A magnetic resonance imaging (MRI) of the head or MRI of the spine.

Other tests that may be done include:²

• Lumbar puncture, a procedure in which doctors take a sample of cerebrospinal fluid (CSF). Doctors analyze the CSF sample to see whether an infection may be causing symptoms. This test helps doctors to measure the pressure of CSF within the skull. It may also allow some controlled drainage of CSF. A lumbar puncture usually is only done for communicating hydrocephalus, which is hydrocephalus not caused by an obstruction.
• Genetic test, which examines a DNA sample to test for abnormal genes or to analyze the number, arrangement, and characteristics of the chromosomes. This may be done if the parents' medical histories indicate the possibility that a baby has inherited hydrocephalus. Genetic counseling is recommended along with this testing.