Sadly I have been in a very similar position to you. I am so sorry that you are now walking this path too 
My daughter was born 6 years ago. I had a feeling throughout my pregnancy that something wasn't right, I told the sonographer at every scan (I had them monthly) and she told me I was being paranoid. She checked my daughters organs and that she was growing and dismissed what I said because, on the face of it everything was growing normally.
When my daughter was born she was taken away before I even saw her, I had no clue what was going on. I won't go into details as it's very distressing, but she was on life support for 2 weeks and there was no hope at all she would get better so we had to let her go.
I blamed the sonographer entirely. I needed someone to be angry at, and she was that person. I wrote to the hospital detailing my concerns and how the sonographer had spoken to me. They replied more or less asking what I wanted to do, did I want to take legal action.
When I thought about it, after the initial intense anger subsided, I realised it wouldn't have changed anything really. By the time my daughters condition was picked up, tests done etc I would then have either had to decide have a late termination or have given birth to her as I did, although I would have been more prepared the outcome would have been the same.
My personal choice was not to take legal action, in reality she didn't do much wrong, and there are hundreds, maybe thousands, of conditions that would be nearly impossible to spot on a scan, but I did complain about the way she talked to me and she was given 'extra training' whatever that means.
Obviously its your choice what to do, but I would say before you make a choice, to speak to someone at SANDS (as a pp has said and linked to) they can really help you work through your feelings and their only agenda is to help do what is right for you if that is legal action they can support you, if not then they will help you however they can.
Whatever you decide to do please be gentle on yourself, there is no right or wrong way to deal with losing your child, you just have to do what gets you through each day 