Truth be told, there never was a "good time" to tell you both. I couldn't tell you before you were married. I couldn't tell you when you were bailing me out of trouble. I couldn't tell you when you had come back from wherever you had vanished off to last time with the Doctor because you had just lost me.

Try and imagine it. Your best friend since childhood pops over for a visit and learns your child was stolen away by weirdos that plan to do horrible unknown things to her. Is that a good time to out and say "it'll turn out just fine, I'm your daughter"? Or even several months later when Amy was still looking out the window, waiting for the Doctor to come back with news?

I know the equivalent of a deathbed confession wasn't much better, but I was going to change into a new person. It felt like the right time to me.