Ultrasound confusion

Travers
So I'm a little co fused as to when to go for an ultrasound. I hear that you can go from when you're 8-12 weeks pregnant once only. I have also read about scanning for down syndrome at 11-12 weeks so is that two separate scans or all in one? Does that mean if I wanted both done I wait for 11 weeks rather than 8 weeks?