Stretch marks??

Question for women who have already had a baby. . . When did you first notice stretch marks?? I know it is quite probable that I will get them because I already have a lot on my boobs from when I was a teenager. I just wasn't sure if it something that will happen nearer the end of my pregnancy or earlier? I feel as though my skin is tight already but haven't really grown a huge amount (I'm 21weeks) thanks!