Its very possible that it wll move out of the way on its own. I had one (3.6cm) detected at my 12 weeks scan when I was pg with DS, that was in a similar position. My OB said he'd do a scan at 36 weeks to see where it was, but that he'd more than liekly go for a trial of labour anyway.
They can change size while you are pg too. Im not sure what happened to mine it couldnt be found at all on my 36week scan so all went ahead fine.