Spoilers, sweetie.

So, this is the timeline for the characters The Doctor and River Song as seen in Doctor Who. A couple experiencing their relationship backwards. The Doctor meets Professor River Song is flummoxed by her knowledge of him, that he has his sonic screwdriver in the future and that she dies. The next time he meets her, she is only Doctor River Song, and so on. Something like this.

But the hot theory is that the mysterious girl in The Impossible Alien/Day Of The Moon is the young River. That she kills the Doctor in the space suit (the first time we saw her, as Professor River Song, she wore a space suit) and is later imprisoned for killing "the best man I ever knew". Doctor River Song, dispite being a crack shot, is unable to hit the occupant in the suit, ending her attempts with the words "No, of course not." How could she kill her past self and create a paradox?

Also, there's also the possibility that the child is the quantum pregnancy of Amy and Rory, able to regenerate (once) after gestating in the TARDIS.

And indeed River Song could be Amy and Rory's child. Pond to River… in which case, the visual looks like this.

It's only going to get more complex.