No, it's a correct use of the word "premature birth". I wasn't being literal as in "when do humans usually get born". I was comparing us to other species. I didn't make up that phrase on my own, it comes from a Dawkins, Matt Ridley, Robert Wright or some other evolutionary biology author...
No, you're missing the point. If a child is born prematurely, with a smaller skull, then any attendant risks regarding the mother dying from complications that might result due to the big-head/small-birth canal issues are lessened. Has nothing to do with twins being born simultaneous.
I know this is an old thread but I had the same question today and Google transported me here.
In Monique's 2nd to the last post above, she mentioned that the cause is unknown, and in her next post she said it does not seem to be a result of "genetic cues". If this implies it's not...