Hello Young matthewd,
First, do you know how long 12 mm is? That is 1/2 inch. I have no idea what is significant and what is not regarding a difference in the length of ones legs. HOWEVER, may I suggest that you make an appointment with your doctor and discuss the issue with him/her.
I commend you for wanting to get additional information, and to do some research on the internet. Me thinks you and I are on the same wavelength on this.
Okay, a question if I may - When you walk does it feel you are walking "straight" with no back and forth motion? No sure how to state this properly. Has any of your friends, or anyone ever commented that it appeared one of your legs was shorter than another?
Stay in touch. Let us know what your doctor says.
I hope others may be able to better advise you.
RichT