This is my first and I had a full and obvious line by 18 weeks (from bbs to pubic bone), but it was a few weeks before when it started to appear below my belly button. I thought it maybe had something to do with skin type - I am fair skinned with blonde hair, but I tan really well. But as some of you ladies are saying you had it with one pregnancy and not the other then it mustn't be that!! Such a weird thing.