Postpartum girdle/cincher advice?

Hey ladies. So I'm looking into using a postpartum cincher/girdle but I'm curious how you knew what size to order before giving birth? I would like to have it ordered and ready to wear after I have the baby but have no idea how to pick a size since I don't know how big I will be exactly? Any advice? Did you just go by pre pregnancy weight or did you wait to order yours till after giving birth? Thanks :)