I would use from now if you are comfortable on it. I've been using one since about 20 weeks I reckon. Had bad pelvic pain and found hip rotations etc to be helpful in easing the pressure a bit.
I've been using one since I was around 16 weeks. It was the only thing that helped with tailbone pain.
You can start using it whenever just try not to bounce unless your over 37 weeks as it can potentially bring on labour I’ve heard 😅
I hardly used mine then started using my ball daily instead of my work chair… got me from 3/5 to 2/5 head engaged in 6 days - start on it asap ! I am actually shocked at how good it moved me along
Now, I've been using mine since 30 weeks