My last one was 20 weeks as well (beside the 3d i paid for out of pocket at 26 weeks) i'm 33 w 4 d now. He said i wouldn't get another one unless they can't figure out what position he is in closer to birth or something isn't right.
I think it just depends on what your doctor wants to do. I've been having an ultrasound each month, next one is tmrw..ill be 34+3
I'm 37 weeks & I haven't had an ultrasound since 20 . My Dr said I'm not getting another one :(