With #3 I just started wearing maternity clothes and let them figure it out, but I did tell MIL about it.

I know you don't want them thinking or saying bad stuff, but it has got nothing to do with them if you want another baby and it is not like you are asking their permission, you are just sharing your happy news with your family.

They will get over it and by the time bubs is born they wont even remember what they said.

Good luck with telling them though.