I had one picked up at my 12 week scan with DS.
It was 3cm, and grew to 6 by my 20 weeks scan. 6cm is actually not that large, my cousns had one the size of a small melon.
They were not worried about mine either, and it was right near my cervix, my OB said they tend to move out of the way. I had a late scan at 36 weeks and they could not see it at all, and nothing came up on any of the scans I had this time wither.