In one of my pregnancies, my progesterone was low and I took suppositories. Unfortunately, I miscarried at 10 weeks. (I went on to get pregnant w/ twins a few weeks later!)
There are basically two reasons progesterone could be low. First, because the pregnancy isn't progressing well (as in my case) and my body was getting ready to miscarry. In that case, progesterone suppositories would not help. There was something genetically wrong with the fetus (we learned after testing) and giving progesterone would not have changed that. Second, some women don't produce enough progesterone to sustain a pregnancy. If you are in that category, the progesterone could help.
Good luck and I wish an uneventful nine months for you!