If your only problem is low progesterone, then the supplement will definitely help...however, it's almost impossible to know whether the levels are low due to your body not producing enough progesterone, or because the pregnancy is a non-viable pregnancy to begin with (and if that's the case, the supplements will not help). Most doctors will prescribe a supplement, though, just as a precaution. It won't hurt and could possibly help sustain a pregnancy.
I've been on progesterone twice during pregnancies. The first time, after getting pregnant naturally, my levels were low and I was put on prometrium. Unfortunately, it didn't help that time and I ended up miscarrying about a week later. The second time, after IVF, I also took supplements (which you have to with IVF), and this time, I haven't had any problems sustaining the pregnancy (I'm now almost 30 weeks).
So hopefully it's just your body not producing enough progesterone on it's own...and if so, the supplement will definitely help!
Best of luck with everything!
Dr.'s have mixed opinions on this. You will find that there are many of us here on progesterone supplements as low progesterone is a common concern in us over "35er's". As to whether or not it has an impact, again, it depends on which OB or RE you talk to. It will help stop spotting though. In some of my very early losses, when it was determined that the pregnancy was ending, they had me stop the progesterone in order to allow nature to take its course. However, even while on it, I was spotting and/or lightly bleeding, so I don't think it stops it completely. I wish you well.