Anything below 10 is considered very low. Mine was 7 with my second daughter, and I had lots of bleeding. I took progesterone and now she is almost 3 and just fine. I did have problems at the end of my pregnancy with severely low fluid and was induced at 34 weeks. Then her sugar was dangerously low when she was born and she spent 4 days in the NICU. I've always wondered if the placenta maybe wasn't as great because of the low progesterone in the very beginning...that could have affected its function as far as producing adequate nourishment to my baby, which could also help determine amniotic fluid levels. My OB said that it's hard to say if the progesterone's low because there's a problem, or the other way around. Levels vary quite a bit, but I was told that they like it to be at least 20, so I would definitely ask for the supplement.
My first pregnancy ended in miscarriage, though I never had my progesterone levels checked. I went on to have a complication-free pregnancy with my first daughter, followed by the difficult pregnancy I mentioned above. After that, I had 2 more regular pregnancies with no problems. My children are now 4, 2, 1, and 2 months!
I'm wondering if you have charted your cycles previously??? My luteal phase is short (10 days), and that is associated with lower-than-usual progesterone. If the egg gets fertilized but doesn't have time to properly implant because of progesterone not keeping your lining in long enough, that can definitely cause miscarriage. However, I did have 3 "normal" pregnancies with no extra help, so even with a short luteal phase it's very possible to get pregnant and for the egg to implant correctly.
Best wishes to you--I've been there, and I know how emotional it is!!!! Let us know what happens!