Great study to link to, Danielsmom!
I know it's overwhelming, but Caryn has collected studies on a lot of common topics and posted them here viewtopic.php?f=28&t=26469
there are over a dozen in the "Dietary and exercise factors and preeclampsia" section, and more in "Body Mass Index and preeclampsia".
My anecdote is that my BMI was 25 before getting pregnant with my first, 23 with my second, and 19 before my third. My blood pressure started going up at 35 weeks with my first, 36 weeks with my second, and 27 weeks with my third. I delivered at 37 weeks with all three of them, so my not-a-doc guess is that for me pregnancy is unmasking tendencies to chronic hypertension and that getting older, despite losing weight, made pregnancy harder on my body... but that's just a guess.