It is generally accepted that poor tolerance to changes in vaginal bleeding associated with hormonal contraceptive use may influence compliance and continuation with the chosen method. However, disparities in the collation and reporting of bleeding data hamper comparison among studies and products. In this review, we systematically assessed MEDLINE and EMBASE for articles assessing parenteral hormonal contraceptives that reported bleeding data based on reference periods as recommended by the World Health Organization (WHO). Overall, 31 studies published between 1986 and October 2007 were included in this review. The use of parenteral hormonal contraception was in general associated with a decrease in bleeding with continued use from Reference Period 1 to 4. However, this decrease was less marked with the combined hormonal depots and both progestin-only and combined hormonal vaginal rings than with progestin-only implants, depots and the levonorgestrel intrauterine system. Overall, reporting vaginal bleeding by 90-day reference periods as recommended by the WHO allows straightforward comparison of bleeding patterns between studies.