How accurate are dating scans?

[PixieCake]

I've split this question out from an earlier thread...

I've just been for my 12 week scan and they have told me I am 15 weeks pregnant. This simply does not work out - I am sure of my dates and even did an ovulation test to check when I could conceive that month.

They must base the measurements on averages, but 3 weeks out seems like a lot. Has this happened to anyone else? If so, did your dating scan turn out to be right according to later scans? Or might they tell me later on that my dates were correct after all.

Can some babies just grow faster than others? 3 weeks faster than average?

Thanks

Oops, LMP!

They're very accurate at dating scans, its later on in the pregnancy that it gets unreliable to tell how big a baby is. I calculated i was 12 + 1 at my 12 week scan and i was spot on, my sister had her 12 week scan yesterday and calculated she was 16 + 3 (the hospital forgot to give her a scan at 12 weeks so had to wait until 16) and again she was spot on x