Flowerchild - thank you - that helps heaps.

And thanks to everyone else for their info too.

Armed with all that info I decided to ring my Fertility Specialist and ask about why the BT is at 13 weeks along with the U/S and was told very very firmly that their evidence is that it the most accurate at exactly 13 weeks and no more. So they weren't gonna budge on the date. (Bummer!)

I find it fascinating that even amongst the few of us who have posted there is so much difference in opinion in our care givers! All I can say is half your luck if you are having tests done at 10 wks or 12 wks. I still have 3 w and 1 day to wait and time is moving at glacial speed for me at the moment!

Good luck everyone.