Sharon, strangely enough I didn't get ANY stretch marks on my BBs until I'd been feeding DD for a while, I didn't notice them until she cut down her feeds (around 15 months) and then weaned - then they suddenly showed! I don't know if it was the skin stretching because it was the nipples that were the itchiest of them all!!
The only thing that helped with the itching (and believe me I tried everything) was I mixed up a jar of moisturiser with a drop each of lavender and chamomile oil.