Long-term progestin-only contraception in humans versus animal models

Abnormal uterine bleeding is the leading indication for discontinuation of long-term progestin-only contraceptives. Histological sections of endometria from long-term progestin-treated patients display abnormally enlarged blood vessels that are prone to bleed in a nonuniform manner. Because it has been complex to attain patients willing to complete long-term studies, and good quality endometrial tissues have proven difficult to obtain, animal models have been used to obviate this problem. In this review, we describe these models including the guinea pig, an animal model we have previously investigated, to assess the mechanisms involved in abnormal uterine bleeding following long-term progestin-only contraception.