Also it seems the foetal heartbeat may not be easily detectable by stethoscope before about 20 weeks, so if the pregnancy is in its early stages even nurse Rory could have failed to detect it.

My hypothesis isn't looking very healthy at this stage. Still-born more like.

Is anybody else having second thoughts about this? I think the scene at the end of the third episode is more consistent with my original impression that the pregnancy scan is unable to produce a consistent result. The reason why is another question, but at least I now find myself unpersuaded by the idea that the oscillation is simply the way the display normally works.

Amy will probably end up having time twins.

One baby human.

One baby human-timelord.

Amy will turn out to be the Doctor's mother.

There's a look Amy gives him at the end of "Day of the Moon", She's lying, she's pregnant and knows it and is afraid he will know it. The scanner can't remember thre baby when it looks away from it because the silents are trying to protect it for themselves. Who needs a few million humans when you can have a Time Lord baby. After the orphanage they know who the mother is too. The reason the Doctor is so concerned is the interest the silents have paid in Amy's baby, and why she would tell him and not Rory. Good question??

I'm still fond of the alternate interpretation of "what he must know, what he must never know." Wouldn't it be an interesting problem for the Doctor if he becomes aware that knowledge of whether Amy is pregnant will lead him into a chain of actions and events that will affect the state of the pregnancy!

But now we have seen it still oscilating in Curse of the Black Spot. So I assume it is not just to do with the scanning.

Considering that the TARDIS is doing the scan, it is obvious to me that the undecided outcome is stemming from alternative timelines not decided yet.

In one, Amy is pregnant, in another, she's not. But which timeline will become the "real" (instead of the alternative) one is still in the flux.

Easy really. And I am really curious to see where each of these timelines lead (which we are probably going to find out throughout the series ).

Timelines, possibly. But I'm leaning towards the preponderances of pocket universes in these episodes.