Anatomy scan

My NT scan is schedule for June 13th, which I will be 13 weeks 1 day. I'm assuming my next appointment will be when I'm 17 weeks (4 weeks from the 13th). I am wondering if they will schedule the anatomy scan for that appointment or if I will have to wait another 4 weeks which would put me at 21 weeks. I'm thinking they will probably push it the the 21 week time frame, and I'll find out in a couple weeks anyway, but it's just been on my mind.