A family member of mine, oh, 20 years ago found out the baby they were carrying had severe hydrocephalus. Aka "water on the brain". They decided to terminate the pregnancy, which they still feel was the right thing to do. I agree with them.
From what I've read on hydranencephaly on Wikipedia, it seems that it may not be that easy to diagnose in the womb. The doctor may have missed the signs when doing ultrasounds. When the baby is born, they may appear completely normal. A newborn's base instincts are governed by the brain stem. So they may still cry, suckle, and generally appear normal. Though symptoms manifest themselves a couple weeks after birth. After a few months of life, it develops into seizures, sensory impairment, paralysis, and severe intellectual deficiencies (duh). Apparently the oldest person with the condition survived to age 33.
It's interesting from a philosophical standpoint regarding euthanasia. This girl is so severely disabled she has zero chance of having anything resembling a life. Yet at the same time, I can understand the parents not wanting to abandon her to death. Not something I would wish on my worst enemy.