I didn't have a scan at 12 weeks

Hillary
So I've seen a lot of posts and talk about the scans that are done around 12 weeks that checks the fluid in the back of their head. 
I have had a few ultrasounds just as a "model" for training techs but my only ultrasound from my doc was at 7 weeks.... I'm 20 weeks now. 
Should they have done a scan earlier ?