Well, my progesterone was 9 with my first child (my son) and I was told that was the bottom edge of normal (9-40 being the ideal range). He was fine, but I did have a lot of spotting for weeks, which as you know is nerve racking. I spotted with my first 4 pregnancies (2 healthy kids, 2 losses), but not this one.
The doc put me on Endometrim and I hemed and hawed forever. I was very nervous about the potential for birth defects, but of course, I was also nervous about losing another baby! So, I talked to the doc and she said the birth defects were only found in the synthetic progesterone and Endometrim is a natural progesterone. (I'm not sure I 100% believe that, but it does make sense and I was ready to be convinced.)
Anyways, this pg, my progesterone has been 44 and 48, so wow. The only other time I had progesterone checked (beside my son) was with the last loss and the doc said it was a 5 and that (insert gruff know-it-all doctor voice) "No pregnancy has ever been supported with a progesterone below a 5." I'm sure you can imagine what I think of him, though....
To complicate your decision, I have also heard that adding progesterone at any point once pregnancy has been confirmed has NOT been shown to save a pregnancy. In other words, if you were going to lose this baby, you'd lose it regardless of if you took the progesterone or not. Also, my understanding is the OTC progesterone cream is not very effective. (And, that would seem to hold true for you since you DO have low progesterone.)
Personally, I wouldn't do injections or anything, but to be on the safe side you could do Endometrim once a day (that's what Zubdelia did). Or even a 1/2 tablet once a day would bring your levels up and at least stop the spotting.
Big hugs to you. I hate these tough decisions.