Basically, you should send an announcement to everyone who would want to hear about the arrival of your little one! This of course includes mostly family and close friends, though you might also choose to send them out to old acquaintances, close friends of your parents…anyone who you think would be interested in your child’s birth.
Don’t worry that you might send out too many. Unlike shower invites, recipients of the announcement won’t feel at all pressured to send a gift (though you might receive a few “congrats” cards in the mail). Announcements are simply pretty cards that let folks know baby has finally made an appearance.
If you're really wondering who you have to send announcements to, it's a different story. Sending cards to your immediate family and closest circle of friends should keep you in the clear. If you aren’t mailing formal announcements to many people, consider covering the rest with an email announcement. They’ll appreciate hearing the good news.