Possibly progestin. Generally, you have a 6 fold increase risk of a clot in the leg or lung. Adding estrogen/ progestin birth control pills increase the risk by between 15 x to 80 x- if you received a factor from one parent or two, respectively. The risk is unacceptably high- more than getting pregnant. The literature seems to indicate that a progestin only pill or an iud without hormone is safer.