What key ingredients should I look for in lotions to reduce stretch marks?

Tretinoin, glycolic. Tretinoin and glycolic acid can be helpful in lotions reducing stretch marks.
It doesn't matter. Stretch marks are determined more by genetics than by any creams. There may also be a factor relating to how big your belly gets. You can certainly use creams if they make your skin feel better but there isn't any one cream that is going to do a better job at preventing stretch marks.