The place we use breaks down the weeks and said: 26-28 they described as best viewing 29-32 they described as good viewing Check the website of the place you are thinking of going to and see if they have a recommended window.
I agree, call the place you are thinking about going and ask their opinion. My tech turned the 3D/4D on for us at our anatomy scan on Tuesday and baby was still super skinny and alien-looking. Definitely want to wait until they put some fat on so you get some of that cute baby-ish look. I could tell that her nose looks so much like DS3's which is kind of fun, but who knows what will change over the next 20 weeks!
Re: 3D/4D ultrasound best time
26-28 they described as best viewing
29-32 they described as good viewing
Check the website of the place you are thinking of going to and see if they have a recommended window.