Endometrial thickness is a commonly measured parameter on routine gynaecological ultrasound and MR imaging. The appearance, as well as the thickness of the endometrium, will depend on whether the patient is of reproductive age or post-menopausal and, if of reproductive age, at what point in the menstrual cycle they are examined.
Radiographic features
Ultrasound
The endometrium should be measured in the long axis or sagittal plane, ideally on transvaginal scanning. The measurement is of the thickest echogenic area from one basal endometrial interface across the endometrial canal to the other basal surface. Care should be taken not to include the hypoechoic myometrium in this measurement.
The normal endometrium changes in appearance as well as in thickness throughout the menstrual cycle:
in the menstrual and early proliferative phase it is a thin, brightly echogenic stripe comprising of the basal layer (figure 1); minimal fluid can be appreciated endovaginally within the endometrium in the menstrual phase
in the late proliferative phase it develops a trilaminar appearance: outer echogenic basal layer, middle hypoechoic functional layer, and an inner echogenic stripe at the central interface
in the secretory phase it is at its thickest and becomes uniformly echogenic, as the functional layer becomes oedematous and isoechoic to the basal layer (figure 2); there is through transmission and posterior acoustic enhancement noted
The postmenopausal endometrium should be smooth and homogeneous.
Normal range of endometrial thickness
The designation of normal limits of endometrial thickness rests on determining at which thickness the risk of endometrial carcinoma is significantly increased.
Whilst quantitative assessment is important, endometrial morphology and the presence of risk factors for endometrial malignancy should also be taken into account when deciding whether or not endometrial sampling is indicated.
Commonly accepted endovaginal ultrasound values are as follows.
Premenopausal
In premenopausal patients, there is significant variation at different stages of the menstrual cycle.
during menstruation: 2-4 mm 1,4
early proliferative phase (day 6-14): 5-7 mm
late proliferative / preovulatory phase: up to 11 mm
secretory phase: 7-16 mm
following dilatation and curettage or spontaneous abortion: <5 mm, if it is thicker consider retained products of conception
Please note that these measurements are a guide only, as endometrial thickness may be variable from individual to individual.
Postmenopausal
The post-menopausal endometrial thickness should be less than 5mm regardless of whether a lady is bleeding, or on Hormone Replacement Therapy/Tamoxifen.
vaginal bleeding (and not on tamoxifen):
suggested upper limit of normal is <5 mm 5
the risk of carcinoma is ~7% if the endometrium is >5 mm and 0.07% if the endometrium is <5 mm 8
on hormonal replacement therapy: upper limit is 5 mm
no history of vaginal bleeding:
the acceptable range of endometrial thickness is less well established in this group, cut-off values of 8-11 mm have been suggested ref required.. Please note that
the risk of carcinoma is ~7% if the endometrium is >11 mm, and 0.002% if the endometrium is <11 mm 8
if on Tamoxifen 3: <5 mm (although ~50% of those receiving tamoxifen have been reported to hav a thickness of >8 mm 7)
Referral to a gynaecologist with view to endometrial sampling is warranted no matter what the endometrial thickness if a woman is experiencing PV bleeding.
If a woman is not experiencing bleeding, and the endometrium is thickened, the guidelines are less clear. Either a repeat good quality transvaginal ultrasound or a referral to a gynaecologist is recommended in this setting.