The major offender from what I have seen is the Motherhood group of stores (Motherhood, Mimi, Pea in the Pod, iMaternity). If you give your name/phone number/address in their stores, you're going to get tons of crap from formula companies. They may have asked if you want to receive "coupons and special offers." During my first pregnancy I assumed this meant coupons and special offers for THEIR CLOTHING. Well, they didn't say that, they just let me assume that (purposefully so, I believe, very sneaky!). The coupons and special offers were actually from Similac, Enfamil, Carnation, Huggies, Pampers, Gerber, book clubs, etc etc, and also included those "parenting" type of magazines that are published by formula companies.
Not sure, but BRU might sell addresses when you register. Or at least share the address with "affiliated companies." They give out magazines like Baby Talk in their store, so if that one starts appearing in your mailbox, I would suspect them.
Tipoff for me that it was Motherhood/Mimi that sold my info...my husband had paid for some mat clothing with his credit card but I remembered them asking for my first name and asking if I wanted those coupons/special offers. Interestingly, the mail arrived addressed to my first name, his last name (I didn't change my name). Busted, Motherhood. The mail also sometimes included information that led me to believe that the sender knew my due date...again, possible culprits included Motherhood (who had asked for that info) and BRU (I had registered).