Hi
For our first child we didn't ask the doctors what we were having, as we had suffered a miscarriage before and we jsut wanted a healthy child.
So many couples in the family find out what they are having and name the baby, so when the baby is born it doesn't feel exciting for the rest they just say oh yes and now the baby girl or boy is here and its already named.
I really don't want to be tempted to ask for my 2nd pregnancy but then again during my scan i may change my mind again...lol
The only reason I was thinking Islamically it may NOT be permissable is as Allah is all knowing and we shouldn't try to look into the future which is Shirk. And I know what you mean at 20 weeks you are looking at the ultra sound and not the future technically.
But I will try to wait as subr is the best.
May we all have healthy children, Inshallah.