I've name changed because I'm outing myself to my family here (hello 
)
Anyway, this is a slightly different question as my baby is already born - she is 9 months old. At the time, and for a while after she was born, I couldn't decide between Cecily and Cecilia and ummed and aahed over it for ages. In the end I decided that I preferred Cecily. However, Dh didn't agree really! Eventually (because we were getting close to the 6 week deadline) he agreed to register her name as Cecilia, and said he would call her Cecily. He lied a little bit because he attempted (for a few days anyway) to call her Cecilia.
Now she is known by everyone as Cecily. We all love it (although DH would probably still say he prefers Cecilia). I am in a quandary whether to change her birth certificate to Cecily or not. I have done some research and I know that this is essentially the same name (like Elizabeth and Isabella maybe), and I know how it originated. I have seen other examples where people have used it as a nn.
I do like the idea (if its a valid one) of giving her the opportunity to call herself Cecilia if she wants in the future. The other children also want to retain Cecilia - they love singing the song to her! But I can't help but think that it is better to plump for one name. And that Cecily is nicer than Cecilia.
Oh I know that I'm over thinking this - but I'm conscious that I can change her birth certificate until she is 12 months old.
What do you think?