Girls, could you please tell me what week is usually for ultrasound an…

Girls, could you please tell me what week is usually for ultrasound and screening at 2 trimesters? I read online that it's 19 weeks, mine falls right on New Year's Eve and then holidays again, so I think maybe I can do it earlier? (Of course not now, definitely discuss with the doctor, just she's off until December 21st, but interest takes its own).