I had low progesterone very early in my pregnancy, my ob put me on Prometrium right away I take 200mg twice a day, I am now 12 weeks and Ive seen the heartbeat 3 times... my ob said to take it 2 more weeks then I can stop, my placenta will take over. good luck
I'd like to add that sometimes a miscarriage is inevitable and progesterone supplementation will do nothing but prolong what will eventually happen. This is usually true in the case of a blighted ovum or molar pregnancy. But a perfectly healthy little embryo should survive with progesterone supplementation. You can stop taking supplementation during week 10-12 of pregnancy.
If your progesterone gets too low, you can and will miscarry. Progesterone is necessary in keeping the uterine lining thick, and to keep your body from thinking it's time to menstruate.
I had progesterone levels that kept dropping with my last pregnancy. Once I started bleeding they finally gave me progesterone supplements. But it was too late. My progesterone sky-rocketed into the hundreds whereas my HCG was already dropping (after it had previously been rising beautifully). A sonogram showed "fetal demise"- an embryo without a heartbeat.
I hope that information helps. If your progesterone is below 20, don't be afraid to ask for progesterone supplements!