Ds i found out because I knew that my grandfather wouldn't live to see him and as Grandfather and ds are the only men in my family at all and one was born 1918 and one 2001 I realy wanted one to know about the other.
With dd I wanted us to both have a surprise but then the consultant that did the amnio gave the game away to dp during the scans and amnio itself. Didn't say anything direct just said what did we want and I said i thought we were having another ds and she said wouldn't one of each be nice though? Said in a way that convinced dp it was a girl. But he knew how much I wanted the surprise even though I was really desperate to know so never told me he knew until the day after the birth.
That was a bit longer than i intended, sorry. 
Hope it makes some sense.
I'd never want to know again though it's way more fun having the surprise i think.