Top normal linear measurement is 9.
8 cm: http://www.ajronline.org/doi/abs/10.2214/ajr.184.5.01841510
3 dimensional measurement is more accurate, but 13 cm is a little large. Clinical importance is unclear and further medical work up seems advised, but may be unrewarding as this is non specific.
If this were me, I'd follow it with serial U/S (maybe repeat in 2-3 mos.) so you don't worry.