See my blog on baby shower invitations
Some merchants having baby registries will provide you with cards to include with the baby shower invitations as an extra touch. Special instructions (for example, meal particulars and whether or not gifts are expected) should also be noted along with RSVP information. Make sure you give your guests enough advance notice by sending the baby shower invitations well in advance of the date. Guests will need enough advance notice to set aside time for the shower as well as purchase an appropriate gift. A longer lead time also gives you leeway to invite alternates if any of the original invitees decline.