I'd never heard of them until I was on HB in the early months before we started TTC, but they've become more "common" around our circle of friends in the last few years as we've each had kid(s), etc.

DH gifted me a beautiful DY necklace with DS1's birthstone that I wear daily! I don't really want anything this time around though. Maybe stackable rings with all of my boys' initials or something simple that I'll order myself during leave!