99.99% chance those lines were there, BEFORE prewashing. I've always prewashed and have never had the washing process cause a fold line. I have, however, bought quite a few fabrics with those lines already present. I usually check the fabric before having it cut . . . but sometimes forget and then find the lines, when I unfold the fabric for washing.
Sometimes you can see the flaws on the back of the fabric. I would probably end up trying to cut around the lines but that is just me. Good Luck !!
