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Objectives  To compare transvaginal ultrasound measurements of endometrial thickness with direct anatomical measurements and consider the implications of these findings on clinical practice.

Design  Prospective observational study using two modalities blinded to each other's findings.

Setting  Singleton Hospital, Swansea, a medium-sized District General Hospital.

Sample  Forty-seven women admitted for hysterectomy.

Methods  All women underwent transvaginal ultrasound scan to measure the endometrial thickness within 16 hours of surgery. Anatomical measurement of the fresh specimen was carried out immediately after surgery.

Main outcome measures  Agreement between ultrasound and anatomical measurements of the endometrial thickness.

Results  No ultrasound measurement was possible in 15% of patients. When both values were obtained, transvaginal ultrasound measurements were >2 mm different from the ruler measurement in 13/40 (33%) with an obvious tendency of the ultrasound scan to over-estimate the endometrial thickness. The mean difference between the two measurements was −0.8 mm (limits of agreement −7.1 to +5.5 mm). The discrepancy was greater in women with endometrial thickness ≤5 mm (−1.6 mm, limits of agreement −5.7 to +2.6 mm) compared with that in women with endometrial thickness >5 mm (−0.2 mm, limits of agreement −7.6 to +7.2 mm). Kappa statistics showed good agreement between the two measurements in discriminating between thin and thick endometrium in 77% (κ= 0.55). Transvaginal ultrasound misdiagnosed a thick endometrium as thin in 3/40 (8%) and misdiagnosed a thin endometrium as thick in 6/40 (15%).

Conclusions  Transvaginal ultrasonography is of limited value as a screening test for abnormal endometrium in patients with postmenopausal bleeding if the only parameter of normality is an endometrial thickness of 5 mm or less.