I know it wouldn't be exactly ten weeks prior. But aprxt ten weeks?

The date they give is ROUGHLY plus or minus 2 weeks... totally a 4 week window. That's if its done by someone really good at making the guess... it only gets wider if they are less experienced.

As long as you get regular periods. I have polycystic ovarian syndrome and I conceived 2 weeks out of the 4 weeks conception! To be sure you should go over it with your doctor.