i thought the same thing about the age risk thing, cos i'm only 26, but my ob said it's not about age, it's about whether you want to know or not, cos it happens at any age (of course the risk is lower in lower age still)..

anyway i heard that the best timing is 10wks for the BT and 12wks for the US/NTT..
but i went for my US/NTT at just before 13wks with no BT (i was slack organising it), and the doctor said i could still go for my BT even at 13 wks..

and that the US/NTT on its own picks up 75%; together with the BT it picks up 95% (or something like that), that's why they usually send you to both, for greater accuracy/sensitivity..