BF breastfeeding (or best friend or boyfriend depending on context)
BFP big fat positive (refers to a pregnancy test)
BLW baby-led weaning
BRB be right back
BTW by the way
bump Not an acronym, of course. People sometimes post a message saying just "bump" - as a way of getting a thread back into the active conversations list so that more people see it. Any posting will "bump" a thread back up the list.
CAT contact another talker - use the Mumsnet contact another talker faciliity to contact someone - as in "Why don't you CAT me?"