My non-medical guess is that women with underlying fertility issues (in some/many cases which can be unknown/unexplained), are more likely to get pre-e/HELLP. I used to frequent a lot of fertility forums for people struggling to TTC, and pre-e/HELLP comes up on the pregnancy sections very very rarely. So I'm not so sure that there really is a great link.
I have long crazy cycles. Ovulated somewhere around day 100+ with dd and got pregnant, developed HELLP at 23 weeks. Struggled to conceive DS, we did fertility meds (femara ended up working best for me), and then IUI (mostly IUI out of frustration of being unsuccessful more than necessity), it worked our first IUI but can't say for sure that the IUI even helped or we caught the egg the old fashioned way. This third time, I managed to get pregnant while still breastfeeding, I used soy isoflavones for 5 days (similar to clomid, it works the same way for some people), to my surprise it worked and we timed things well and got pregnant the first month we tried, completely on our own. My progesterone was lowish and I most likely would have miscarried if I had not had it tested and supplemented.
Good luck to you! I know TTC can be really frustrating and difficult.
Me (29) DH (30)
#1-Olivia Caetlyn-9-28-09-9-28-09, 23+2 wks, emergency classic c-section, class I HELLP, IUGR
#2- Lucas Oliver (rainbow baby)- April 2011, 36+2 wks, HELLP and pre-e free! (lovenox and LDA pregnancy)
#3-Matthew, late October 2012...mostly normal, 37 wks, (lovenox and LDA again)
My blog: http://www.butterflies-and-rainbows.blogspot.com/