i think it was orci who said that indeed the birth of jimmy boy was early.
that kind of stress could have caused him to arrive weeks early.
I disagree, if only because the attack took place so quickly. Now, if Kirk had been delivered by C-section, I'd buy it. As it is, if we're really conservative and say Pike's "twelve minutes" was counting from the moment Robau died, and said it took, say, a half hour for Robau to shuttle over to the Narada, add maybe ten minutes for the initial attack, that's still less than an hour, which is extremely quick for such things (actually, I don't remember if we first saw Winona before or after the attack started). I suppose we could say that she had the most skittish uterus in history and was induced just by the fact that the Kelvin changed course to investigate the anomaly, but since there wasn't that much lead-time between the second opening appearing and Spock coming out of it, I don't think that adds enough time, either.
My point is, for Kirk to have been born before the Kelvin rammed the Narada, Winona would've had to have gone into labor well before the timeline began to change, so Kirk-Prime was born on the Kelvin, too.