Abberant endometrial angiogenesis has been implicated in abnormal endometrial bleeding in users of progestin-only contraceptives. We believe that a possible cause is an underexpression of thrombospondin-1 (TSP-1). Progestins had less effects on TSP-1 mRNA compared to P, in Ishikawa cells, and this difference in TSP-1 expression by individual progestins may be associated with the irregular bleeding seen in women using progestin-only contraception.