I do not know about the risks of a cyst or fibroid causing a miscarriage but I do know that they should be checked regularly to make sure they don't become too big. I had an ovarian cyst removed in emergency because it had become too big (6*7 cms) and looked cancerous (fortunately it was not) and my left ovary never recovered.
The good news is I was able to get pregnant 3 times with only my right ovary and to finally have a successful pregnancy (I am 24 weeks).
I have 2 fibroids and thought initially they might have caused my miscarriages as well. My OB didn't believe they were the culprit and for now has opted to leave them in. I had no trouble conceiving again, and am now almost 39 weeks pregnant and all is well.
I know after a miscarriage one of the things we want so badly is answers as to why. Often there just aren't any, and sadly miscarriage is more common then most of us realize. In first trimester, the most common cause is a chromosonal abnormality and there is nothing that can be done to prevent that. As we get a bit older, they become a little more common due to the age of our eggs. Fortunately the majority of women who have a loss will go on to have a healthy pregnancy. I am sorry for your loss, and I hope you are expecting again soon. Good luck to you.
I just completed a pregnancy with a fibroid 5cm and a cyst, but everyone is different so its hard to say.
Baby Dust to you,
Joanne