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Endometrial thickness is a commonly measured parameter on routine gynecological ultrasound and MRI. The appearance, as well as the thickness of the endometrium, will depend on whether the patient is of reproductive age or postmenopausal and, if of reproductive age, at what point in the menstrual cycle they are examined.
The endometrium should be measured in the long axis or sagittal plane, ideally on transvaginal scanning, with the entirety of the endometrial lining through to the endocervical canal in view 10. 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 hypoechoic myometrium or intrauterine fluid 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 (figure 2).
- in the secretory phase it is at its thickest, up to 16 mm 10, and becomes uniformly echogenic, as the functional layer becomes edematous and isoechoic to the basal layer (figure 3); 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:
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.
Endometrial thickness can decrease with long-term combined oral contraceptive pill use 12.
The postmenopausal endometrial thickness is typically less than 5 mm in a postmenopausal woman, but different thickness cut-offs for further evaluation have been suggested.
- 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
- 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 HRT / tamoxifen have been reported to have a thickness of >8 mm 7,13)
If a woman is not experiencing bleeding, and the endometrium is thickened, the guidelines are less clear. Either a repeat transvaginal ultrasound or a referral to a gynecologist is reasonable.
Endometrial thickness is well assessed on MRI. Measurement should be taken at a mid-sagittal slice, similar to the ultrasound assessment plane.
- T2: normal endometrium is homogeneously hyperintense regardless of the phase of the menstrual cycle or menopausal status and well outlined by the low signal myometrial junctional zone
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