Belfie, mine was within 1.5cm of the os and had moved to 3cm away. Apparently something like 25% of women get "previas" that really aren't, and a lot of women get freaked out unnecessarily by their OBs who get all dire about c-sections and stuff. The vast majority of them move up as the uterus expands. Interestingly, they "move up" by the cells at the bottom of the placenta dying off and the ones at the top growing more rapidly. It's probably not as good a statistic for women who have a full previa, but still not hopeless.

If there's no bleeding, and especially if the "previa" is marginal, there's no reason to modify your activity in any way at all, although i chose to abstain from vigorous pelvic activity until I had been reassured, if you know what I mean!