Re: Doctor Who?...
I'm guessing that the explaination for Amy's positive/negative pregnancy scan will be quite simple. The future is not written, it is in a "constant state of flux" which was mentioned in the first episode of this series. What if the Doctor's future involves him making a decision to go back and change Amy's past? One choice would mean she is pregnant in the present, and the other choice means she is not. Since he has not yet made that choice yet, her current pregnancy is also in a state of flux.
That's what I meant when I said pregnant and not pregnant at the same time and that the foetus was possibly not in the same timestream as her.