See, I've wondered about that. Richie and Mike saw IT come when the world was still new. Back before there were men, even. If you take into account the fact that IT sleeps for twenty-something years, that would STILL be an incredibly long gestational period. She would have been gravid for millennia. The timing just doesn't make sense to me. Also, when the Losers first fought her as children, she wasn't far enough along for anyone other than Stan (maybe) to notice her "delicate condition". If she'd been pregnant for millennia, she would have been a lot more obviously pregnant back in '57. Especially when you consider just how pregnant she was when the Loser's come back in '85. To go from "just barely showing" to "ready to pop" in just twenty-seven years? When she's been pregnant for millions? The math doesn't add up for me.
But, if "she" was in fact hermaphroditic, like some snails, then her pregnancy wouldn't be such a surprise. This is, in fact, the simplest and most logical explanation.
But then we have the Crimson King. More of a malignant were-spider than a shape-shifting demon from Outside, but perhaps IT and the Crimson King were similar enough to produce offspring. How did they meet? Not a clue. In one of IT's twisted dreams maybe? Who knows? It's the least likely probability, but just imagine if it happened...and one of their spawn survived. What would THAT have been?
