Someone asked me a question in the comments awhile back as to whether I am on blood thinners and if I had the underlying tests done. So all the tests were done after I had preeclampsia and HELLP in 2005 and all came back negative for any condition or gene mutation. With my new perinatologist she also tested homocysteine levels (which has something to do with the body not processing folic acid right) and these were normal. I believe if you have the MTHFR gene mutation - which many people do - sometimes your homocysteine level is high (possible link to preeclampsia and increased stroke risk) Anyway truth is I was never tested for MTHFR and my perinatologist's reasoning was that mutation is common in Caucasians and what really matters is whether your homocysteine level is high or not.
So...being there were no known underlying causes we went with low-dose baby aspirin rather than any of the hardcore blood thinners like Lovenox used by many in post pre-e pregnancies. I started the LDA before conception. I know some folks are more comfortable waiting til after 12 weeks and this is something to consult a doctor about - I only offer my take. But I figured if preeclampsia is something that starts to form right at conception and is essentially a problem with bad implantation of the placenta, then I wanted my blood to be in as good a condition as possible from that moment.
Of course it would be nice to not take anything during pregnancy but it's a risk/benefit analysis at end of day. I was encouraged by a smallish study out last year showing no increased risk of brain bleeding in babies whose moms were on LDA: http://www.businessweek.com/lifestyle/content/healthday/634272.html