Pathophysiologic features of "thin" endometrium.

To characterize pathophysiologic features of a "thin" endometrium. A prospective observational study. University Hospital and City General Hospital. Patients with normal-thickness endometrium (Normal-Em group: endometrial thickness >or=8 mm; n = 57) and thin endometrium (Thin-Em group: endometrial thickness <8 mm; n = 17). Blood flow impedance of the uterine radial artery (RA) was assessed as resistance index (RI) by transvaginal color-pulsed Doppler ultrasonography. The area of glandular epithelium, the number of blood vessels, and vascular endothelial growth factor (VEGF) expression were examined in the midluteal-phase endometrium. The RA-RI in the Thin-Em group was significantly higher than in the Normal-Em group throughout the menstrual cycle. Endometrial thickness was significantly correlated with RA-RI. Growth of glandular epithelium, the number of blood vessels, and VEGF expression were significantly lower in the Thin-Em group than in the Normal-Em group. A "thin" endometrium was characterized by high blood flow impedance of RA, poor epithelial growth, decreased VEGF expression, and poor vascular development.