Where have they suddenly sprung from?
They make my teeth itch. Of the three I've been invited to, one had 60 invitees (presumably to get 60 gifts, because some of the people invited were really only well off aquaintances) one had a bloody gift list with the invite and the other was from my old slimming club leaders daughter that I'd already given a travel system to. 
Surely, if you want to buy somebody a baby gift you do it after the baby is born, and don't need to be summoned to spend an afternoon with people you don't know playing weird baby related games and trying to guess the birth weight?
And lastly, surely to god if you were having one you should invite you mum, who looks after your existing child 3 days a week while you work? narrows eyes at sil