I don't think I have ever heard that fingering causing you to loose a baby. Most dr,.'s say it's ok to have sex in the first trimester so I assume that fingering would be ok too, unless your dr. says not to of course.
since you are so early into pregnancy, a miscarriage is still quite possible. (don't wan't to scare you but I thought I should just say anyway) Anyway, really there is not much that you can do....baby is pretty well protected in there so as long as you aren't doing anything like lifting things that are too heavy etc then you should be fine. Fingering can't cause a miscarriage.