As soon as we found out we were having a girl (through Amnio) we decided on Matilda as her name and always referred to her as Matilda during my pregnancy but only DH and I, it was a secret to everyone else. When she was born we never even considered changing it, it was her name!

Suggestions:

Mason
Archer (Archie)
Fletcher
Eden
Sebastian

Go on use Fletcher... I can't because our surname starts with Fl... but I love it.

And for what its worth I have always thought Spencer was a cute name.