Stopping progesterone

I have had two previous miscarriages due to low progesterone. Both ending at about the 6 week mark. I started progesterone right after ovulation with this one and I am currently 12w as of tomorrow. I am on 200mg suppositories, twice a day.  My dr wants me to stop at 12w but I am scared. Is there a test you can take to see if the placenta has taken over? Has anyone continued progesterone past the 12 week mark? Any advice or experience would be so helpful! Thank you!