Other. Mine are from puberty. I went from being a little girl to being "womanly" very quickly. Fortunately they are very faded. I lucked out and have none of my belly from my pregnancies.
Three babies, including two over 9-lbs. No stretch marks.It's a genetic thing. If your mother had them, you're more likely to have them. My mother birthed 5 babies and has nary a stretch mark to show for it.