Telling family you are pregnant protocol

So it's our first pregnancy and we wanted to wait until the end of the first trimester. That lands right around my husbands birthday, so we were going to have a party and announce to our family there. I actually have a cute plan of hiding clues around the house and then his gift being dad related so all the family will find out. 
My question is, is it rude to not tell our parents separately ahead of this little event? I'm worried they'll be offended if they find out with everyone else and would like to tell them about a week or so before, but I don't think (nor does my husband) that my mother-in-law will be able to keep it secret from the rest of the family. I'm just not sure what the proper thing is...