If you got stretch marks, what week did they start showing up?

If you've already had a baby (or are pregnant and already got them) and got stretch marks on your belly, what week did they start? I'm only 15 weeks, and I've only gained about three pounds, but I feel like I keep seeing some weird tiny redness on my stomach. Perhaps I'm seeing things or it's something else ... but just wondering, how many weeks along were you when you started to get them and about how much weight had you gained at that point?