Prometrium is supplemental progesterone. Progesterone is an essential hormone during pregnancy and some women don't make enough, which results in a miscarriage. I miscarried my first pregnancy at 8 weeks after beginning spotting at 6 weeks. When the same spotting occurred with my second pregnancy, my doctor also checked my progesterone levels and found that they were low. She went ahead and prescribed me the progesterone, although she said the pregnancy may still miscarry. Low levels of progesterone are usually a sign that the pregnancy is not viable, but every once in a while, like with me, my body does not make enough and it is the low level of progesterone that causes the miscarriage. The bleeding stopped immediately after my first dose and I took it until 12 weeks when the placenta is fully formed and takes over the role of producing progesterone.
With my second baby (third pregnancy), my doctor monitored my progesterone levels twice per week once I was pregnant and sure enough, at about 6 weeks, I started spotting and my levels didn't rise as much as they should have. She immediately prescribed progesterone and the pregnancy was fine.
Now with my third, my doctor didn't bother checking the levels--just prescribed me progesterone the moment I found out I was pregnant. The likelihood is that I would have the same issues I had with my other pregnancies, but even if I did not, the doctor said there was no harm in taking the progesterone and your body does not use any excess. I am now 15 weeks and the pregnancy is going well.
Finally, as for bad side effects, I have not had any--your body should be making the hormone anyway so it is completely natural. On the good side, my nausea actually let up quite a bit once I started the progesterone. It seemed as though I felt even worse when my levels were lower than they should have been.
Good luck and be glad your doctor is trying other means to stop you from miscarrying. My first doctor with my first pregnancy was way too relaxed about the miscarriage and said one or two miscarriages during the first trimester are very common. She refused to do any additional tests. Needless to say, with my next pregnancy, I switched doctors and am so glad we were able to figure out the problem. We have two very healthy little girls and are on our way to the next!
Best of luck and hope this is your problem as well, because if it is, it is very easy to handle once it is diagnosed!