I would not want to be within 20 miles of her when she goes into labor. I also think if she ever had a baby she'll end up blowing it up or eating it. It would be exactly the same as when Yuuka adopted that third world kid.
I can only assume that this is what Dr Z was referring to: http://i38.tinypic.com/rqxil3.jpg Makes me laugh, that one. I'm just sort of curious to know what the context was that prompted the anon to say the line that inspired it in the first place. Because seriously, what?
This isn't a declarative sentence. She's not actually stating that she has the baby inside, but is rather asking "if she has his baby inside her stomach." I think...
and if we use your translation, it will become something like ~ちゃんがいるんだよ・知ってた? instead of いるの?
As Maxwelone said, it's not a declarative statement disguised as a question, though that does happen regularly in Japanese.
Put into context with the image that goes previous to this (post #422470) it becomes clear that Flan is echoing the answer as to why she began lactating.
Without the parent post's context (which I didn't check, I realize), it definitely could have been a declarative with inquisitive tone. I've spoken to Japanese people who use it like that; it's a tone-of-voice thing that becomes vague in textual form.
With said context, though, you're right, and I'll bow out of that.
However, 知ってた?= "you know?" is too literal in thinking. "You know?" is a colloquial, nigh-meaningless sentence-ender in many English-speaking places. Think of it somewhat in the same vein as "ね?" rather than the more literal "知ってた?"