I don't normally subscribe to nicknames for things, but I do like the term rainbow baby for this reason:
Josie was my very first baby. The one that comes after her will not be. He or she will be my second baby. Calling my new baby my "rainbow baby" makes him or her a first as well, because the last thing I ever want is for my future son or daughter to feel that he or she was less special than Josie because he or she came along later. No other child will ever replace my baby girl, but another baby WILL lift the clouds a awful lot - I long to hold a living child in my arms and I will have this longing until another comes along. That child will be a rainbow, and I love rainbows - I think they're gorgeous, especially when flying (sometimes they can be perfectly round then because you're so high up!).
So, I do actually like the term "rainbow baby" - it makes me feel hopeful and positive about the future and I look forward to my rainbow baby being here - not my first, but my rainbow
But that's my own personal opinion though. *hugs* XXX