Just moved to general pregnancy...

I had one. For me it was about having a celebration that I was going to become a mum very soon. I think it's almost like a birthday party for the baby before they are born. Afterwards you are too tired & busy for everyone to come over at once to celebrate the birth so you do it beforehand.

Maybe if you're worried word the invitation to that affect - that you want to celebrate the new baby but will be too busy once s/he is born?