I have a very faint line that goes from the pubic bone to the sternum. Its has a kink in it where it reaches the belly button, otherwise its pretty straight.

And I got told they appear as a "landing strip" for bubs to find the boobs easier, as they can only distinguish between light and dark. Hence why if you are darker skinned, then your line will be darker again. I also got told it appears in ~75% of women, although the poll doesn't seem to reflect that...