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A functional model for progestogen-induced breakthrough bleeding.

During the past 5 years, a number of important advances have been made in our understanding of the mechanisms of sex steroid-induced breakthrough bleeding (BTB). These observations suggest that superficial endometrial vascular fragility may be the mechanism underlying BTB, and molecular changes in the microvasculature as well as hysteroscopic observations have supported this hypothesis. This paper aims to present a unified picture of our current understanding of BTB, particularly that associated with progestogens, to indicate current gaps in our knowledge and possible directions for future research.