20 weeks is usually pretty clear.
At both my 18wk scan with DD and 19 week scan with this one we were able to see very very clearly the gender.

With this one my OB told me tentatively that we were having a girl at 14 weeks. I took this with a grain of salt as he wasn't sure and it's a bit early to call at that point.
Turns out he was wrong and we found out yesterday this baby is a boy! There was no doubt, we could see it very clearly