We noticed mine emerge from the belly button down a couple of months ago. It's pretty faint. I'm very pale so it came as a shock when it started - I didn't think I'd get one. I also have a line going upwards over my belly but it looks more pinkish and this has only been noticable for the last couple of weeks so it could be a stretchmark (not sure yet).

We are 100% sure we are having a boy (amnio) so it will be interesting to see if the line develops further or if it does just end up being a stretchmark.