Ok, is the ultrasound a Nuchal Translucency Scan? That's the ultrasound done in conjunction with a bloody test (BETA HCG & PAPPA) - testing for Downs Syndrome. If it is, you must be between 11wks 3days & 13wks 6days when you have the ultrasound. It would be most optimally assessed in the middle of those dates which would be 12wks.

The other reason you may be requiring a scan at 12 weeks is to firstly make sure the pregnancy is viable & secondly to get your EDD (according to size & when your LMP was etc).

I would check with your GP or Obstetrican or whoever referred you for the scan as to what type of Ultrasound they have ordered for you to have. What does your referral read?