Several readers are making this point:
As much as I share some of your suspicions, I think you are reading this email wrong. The critical passage:
I let Trig's mom have an exceptionally comfortable pregnancy so she could enjoy every minute of it, and I even seemed to rush it along so she could wait until near the end to surprise you with the news – that way Piper wouldn't have so long to wait and count down so many days …
“I even seemed to rush it along so she could wait until near the end to surprise you with the news” could refer to the fact that Palin didn’t seem pregnant until close to the very end. It was as if she went from not pregnant to seven months pregnant overnight. As a result, she didn’t have to disclose the pregnancy until very late, meaning that the period of Piper’s anticipation was shorter. This could all be garbage (things people write in the voice of God usually are), but it is not smoking gun evidence that Palin knew she would deliver prematurely.
That would make sense, except that "rush it along" doesn't mean "keep it hidden for a long time and then speed it up at the very end," does it? In fact, it seems to mean the opposite.
I know it's hard parsing Palin's logic.
But presumably she's talking about the time after she had told the kids and before she gave birth – that would be the time period that Piper would be waiting impatiently through. In Going Rogue, Palin says she told the kids before she finished composing the email-from-God in March. So Palin must be referring to the period between some time in March and April 18. She is somehow predicting a premature birth – despite also telling us in Going Rogue that her first instinct when going into labor was "It can't be. It's much too early." Yes, I know: the contradictions in her stories are legion. Most women's accounts of pregnancies are less, shall we say, convoluted.
For what it's worth, Trig's given birthweight was at the low end of the normal range for a full-term child with Down Syndrome.