How do birth control hormones cause increased clotting risk?

I know that birth control pills increase the risk of a clot, and I know it's because of the hormones in them (so please don't tell me that estrogen causes clots, I know this.) What I can't find information on is -how- estrogen causes clots. Do they increase the number of platelets in the blood, or do they make the blood thicker in some other way?





My platelet count has always been high, and it's recently (since I've started birth control) gotten higher. I know that makes it riskier to take combined contraceptive pills, but are the pills themselves causing the high count?





I do plan on discussing this with my doctor, but I won't see her for a few weeks and I'm curious to get an answer now.How do birth control hormones cause increased clotting risk?
I found your question to be very interesting so I did a bit of research. This article I'm linking to seems to indicate that it's due to estrogen causing an increase in platelet activity rather than due to estrogen causing an increase in platelet formation:


http://www.sciencedaily.com/releases/200鈥?/a>





This article below explicitly says that the reason for the clotting issue is not well understood: http://circres.ahajournals.org/cgi/conte鈥?/a>


So I don't think you'll get any definitive answer, because no one knows it yet.





However, it does seem that, per the first article, oral estrogen causes clotting while other routes of estrogen do not. this would suggest the Nuvaring would be a better choice for you as birth control--but it's not supported by any definitive studies right now.





I found a couple other articles that suggest platelets are more reactive due to estrogen, because platelets have estrogen receptors on them:


http://www.nature.com/ki/journal/v38/n6/鈥?/a>


But it's only estrogen receptor beta, not estrogen receptor alpha.