I noticed my line on the 15th week of pregnancy. It runs from the pubic to halfway to my belly button. Above my belly button I noticed that it also run up, but it is really light. The bottom half is a bit darker and can notice it much more than the top.
Linea Nigra can show up any time by: Anonymous
It is different for every woman. Most often it appears in the second or third trimester. But I know someone who has been pregnant twice and both times she had linea nigra as her very first symptom, before she even missed her period, just like you are saying you had. So your dark line sounds perfectly normal but if you are worried at all you should talk to your OBGYN.