The type of progestin in combined birth control may modulate the risk of VTE. Studies have found that combined birth control pills containing newer-generation progestins such as desogestrel, gestodene, norgestimate, drospirenone, and cyproterone acetate are associated with a 1.5- to 3-fold higher risk of VTE than birth control pills containing first-generation progestins such as levonorgestrel and norethisterone. However, although this has been apparent in retrospective cohort and nested case–control studies, no greater risk of VTE has been observed in prospective cohort and case–control studies. These kinds of observational studies have certain advantages over the aforementioned types of studies, such as better ability to control for confounding factors like new-user bias. As such, it is unclear whether the higher risk of VTE with newer-generation birth control pills is a real finding or a statistical artifact. Androgenic progestins have been found to antagonize to some degree the effect of estrogens on coagulation. First-generation progestins are more androgenic, while newer-generation progestins are weakly androgenic or antiandrogenic, and this might explain the observed differences in risk of VTE. The type of estrogen also influences VTE risk. Birth control pills containing estradiol valerate are associated with about half the VTE risk of birth control pills with ethinylestradiol.