My OB does them at the A/S, so around 20 weeks. DS's was at 18.5 weeks and he looked pretty skinny but still looked like himself- it's fun to compare his NB pics to his u/s pics. This time around, I was 21 weeks and it was just ok- DD kept covering her face with her hand. They were actually able to remove her hand from the pic but then it looked kinda funny because she was sucking her fingers so her chin was sorta down and her face was clearly making a sucking motion but the fingers were gone. She just wasn't as cooperative (hope that's not a sign of what's to come lol) but still a good size/week for the u/s.