Hmm, I'm not sure I even knew about "viability date" with my first. Maybe I did and memory has just drawn a veil, lol. But that's cool!
Pompey what was your original due date? I find the changing of due dates very strange. I guess maybe it makes sense for women who really don't know when they conceived, but if they try to change mine at any point they're gonna have a fight on their hands because I'm certain of my dates. Anyway, I find out sex on Sept 1, so one day before you, yay! This pregnancy is definitely different for me too, so maybe it's a boy. I kind of feel "girl" though.
Keep us posted Rose!
I dont know if you remember, but my original original due date was 15th Jan, but where I ovulated on Day 23, I changed it to 9 days later myself because I knew when I conceived. However, the scan has showed to be 6 days ahead, so now it's the 18th Jan. I am totally confused by the whole thing to be honest as like you, I know what I ovulated and when this baby was conceived. However, I'll take it because it's 6 days sooner I'll be at my due date and meeting this little one

It's exciting our 20 week scans are a day apart!