K.M.
I do not have personal experience with hydrocephalus, but had a similar experience with my 20 week ultrasound with my youngest (it also pointed to the possibility of Down's or Edward's). She had a choroid plexus cyst (a fluid pocket in the brain), slight calcification on the right ventricle, and her stomach was distended for over 24 hours (we had back-to-back ultrasounds Thursday and Friday). They were concerned about Duodenal Atresia, which is where the stomach and duodenum are not properly connected. I was faced with the possibility that my baby had Down's and would need surgery immediately after birth if she did have the other condition.
Fortunately, for me, my father has been with a laboratory for years and he was able to talk me through everything that I needed to do. I discussed with the doctor what would need to happen before birth, for surgery if necessary, care that would be required. I also turned to the internet and my family and friends. The more people who know what you are going through, the better prepared everyone will be.
My husband and I opted for an amniocentesis and it ruled out any genetic anomalies. Her stomach returned to normal size, and all ultrasounds and fetal ECG returned normal results. I did not have to go through the daunting process of knowing that my daughter would have surgery in 18 weeks (we were cleared at 22 weeks), but I was much better prepared when I armed myself with knowledge.
Drink in as much information as you can, find a support group now and attend meetings, discuss every possibility (within reason) with your doctors. Learn everything you can from the internet, books, etc., and remember that hydrocephalus is no longer an automatic vegetable sentence. Shunts can do amazing things nowadays and your baby has a much better chance of a normal life because of our technology and your desire to make everything better for him.
Best of luck and keep us informed. BTW, our daughter was born a healthy 7 lbs 11 oz at 40 weeks, with all functions completely normal. She is now almost 7, in the first grade and an avid reader with a wonderful imagination.