Abstract: Background: Abnormal uterine bleeding (AUB) is quite common, one third of women have it and there are women in whom no cause is detected that explains it. To date no reported histological findings that justify the bleeding are published.

Objective: To describe the hitherto unknown cause of AUB, which consists of markedly abnormal vessels in the endo myometrial junction of the uterus (EMJ) that we call Uterine Vascular Hyperplasia (UVH). Method: 520 uteri were reviewed between October 2022 and August 2024, 39 without macroscopic alteration that justified the AUB. Characteristically they have more than 3 vessels in balls in the EMJ, where the number of vessels, outer diameter, thickness of the vessel wall and diameter of the lumen were evaluated. The control group (CG) of 39 uteri from cases without UAB.

Results: All cases with UAB had abnormal vessels in average number of 12.2 VS 3.0 CG, external diameter: 187 microns ((μm) with UAB VS 24.9μm in the CG, the vessel lumen: 71.3μm UAB VS 13.0 μm of the CG, wall thickness: 57.9μm UAB VS 5.9μm CG, with p= <0.0001 in all parameters.

Conclusion: It is described for the first time the finding of abnormal vessels as a possible origin of idiopathic or unknown cause of UAB that consists of the presence of abnormal ball vessels with an increase in the quantity, size of the external diameter, wall thickness and diameter of the lumen that compared with normal CG is between 4 to 9.8 times greater, which explains the UAB.

Keywords: Uterine Bleeding, Abnormal uterine Bleeding, PALM-COEIN, Unknown uterine bleeding