This is also a natural pregnancy for me and my RE also said I could stop the progesterone (I was on Endometrin -100 mg 2x a day) around 9 weeks. My P4 was last tested around week 8 or so and it was 17. My RE said it was actually a lot higher since Endometrin didn't show up in b/w and would be safe to stop.
I just wasn't comfortable stopping at that point and I still had a few weeks of Endometrin left, so I finished what I had left. I tapered down to 1x a day around week 10-11 and took my last dose when I reached 12 weeks. Even though my RE and my OB said it was okay to stop, I personally wasn't comfortable stopping until the 12 week point. Even then, I was nervous, but didn't have any P4 left, so had to trust that all was ok.