I have had a low progesterone level for most of my life. I've been married almost 13 years, and in that time had 1 pregnancy that ended in miscarriage (1995) and 1 pregnancy that is going to full term (due the end of this month).
In the past 12 years I've never used birth control and yet never got pregnant except that one time, and my progesterone has always been low.
We did try fertility treatments (Clomid, hCG shots, etc...) but nothing worked.
My progesterone levels with this pregnancy were "a bit on the low end" and I was actually put on progesterone suppositories for the first trimester of the pregnancy, just in case.
Not sure if this helps you at all, but there it is.
Good luck!