"Oral contraceptives with levonorgestrel and a low dose of estrogen are associated with the lowest risk of venous thrombosis [blood clots] and are therefore the safest option," says Astrid van Hylckama Vlieg, PhD, a research fellow at Leiden University Medical Center in the Netherlands, and the lead author of one study.
''Our study shows that there is indeed a difference in the risk of venous thrombosis associated with different types of progestins and different doses of estrogens available in the contraceptive," she tells WebMD in an email interview.
Similar results were obtained in the second study, evaluating Danish women. Both studies are published in the online edition of BMJ.
Even with differences in risk, however, a U.S. expert who reviewed the studies for WebMD points out that the differences are small and the overall risk of getting a blood clot from the pill is very low to begin with.

