The only real reason they send you for a scan before 12 weeks is for a dating scan. Because once babies are over 12 weeks they start to grow at different rates dependin on genetics.
It's only since having my DS they seem to do it with every pg. I only had it done with my girls coz of my severe morning sickness & with a pg in between them coz I had no idea of my dates. Didn't find out til I was 8 weeks.
It is not needed & not compulsory. The 18 week scan - morphology is more important.
My 18 week scans were all done closer to 20 wks & with DS I had to have a 2nd one coz he was stubborn & wouldn't move at 24 weeks - it still took about an hour & lots of bending, touching toes & lying on my belly to get him to move!