Endometrial Thickening
Endometrial thickening describes a uterine lining that measures thicker than expected on ultrasound or MRI. It is a measurement rather than a diagnosis, and its meaning depends almost entirely on two things: whether a woman has been through menopause, and whether she is bleeding. In a woman who still has periods, the lining changes thickness dramatically across the cycle, and a measurement that would be alarming after menopause may be entirely normal at a particular point in the month. After menopause the lining should be thin and stable, so thickening carries more weight—particularly alongside bleeding, which is the single most important accompanying symptom. Most causes are benign, but the evaluation exists to identify the minority that are not.
What is it?
The endometrium is the lining of the uterine cavity. During reproductive years it is a dynamic tissue, rebuilt and shed with every menstrual cycle under the control of oestrogen and progesterone.
Understanding its normal behaviour is essential to interpreting a thickness measurement, because the same number means entirely different things at different points in life.
Immediately after a period, the lining is thin—often only a few millimetres. Through the first half of the cycle, oestrogen drives it to thicken and proliferate. Around ovulation it typically measures somewhere in the range of six to ten millimetres and takes on a characteristic layered appearance. In the second half of the cycle, progesterone converts it into a secretory lining prepared for implantation, and it reaches its greatest thickness—commonly up to about sixteen millimetres and sometimes more—appearing uniformly bright on ultrasound. If pregnancy does not occur, hormone levels fall and the lining sheds.
This means that a measurement of, say, fourteen millimetres in a woman about to have her period is entirely normal, while the same measurement in a woman ten years past menopause is a finding that requires investigation. It also means that when a scan is performed matters: for assessing the lining in a woman who still has periods, the days immediately after a period give the clearest view.
After menopause, oestrogen production falls and the lining becomes thin and inactive—typically under five millimetres and often considerably less. Because it no longer changes cyclically, any thickening represents something that requires explanation.
The causes of a genuinely thickened endometrium span several categories.
Endometrial polyps are among the most common. They are localised overgrowths of endometrial tissue projecting into the cavity, usually benign, and often causing bleeding between periods, heavy periods, or postmenopausal bleeding. They can also affect fertility by interfering with implantation. A small proportion contain precancerous or cancerous change, and this risk is higher in postmenopausal women and in those who are bleeding.
Endometrial hyperplasia is thickening caused by excessive proliferation of the glands, driven by oestrogen acting without the balancing effect of progesterone. It is divided into two categories that matter greatly. Hyperplasia without atypia carries a low risk of progression to cancer and is usually treated with hormonal therapy. Atypical hyperplasia, also called endometrioid intraepithelial neoplasia, involves abnormal cells and carries a substantially higher risk—a significant proportion of women diagnosed with it already have an undetected cancer present, and more will develop one if untreated. This distinction determines management entirely.
Endometrial cancer is the diagnosis the evaluation exists to exclude. It is the most common gynaecological cancer in developed countries, and its incidence has risen alongside obesity. The reassuring context is that it usually announces itself early through bleeding, so the majority of cases are diagnosed at an early and highly curable stage. The unreassuring corollary is that bleeding must actually be reported and investigated for that advantage to hold.
The unifying thread through hyperplasia and most endometrial cancers is unopposed oestrogen—oestrogen stimulating the lining without progesterone to balance it. This explains the risk factors. Obesity is the most significant modifiable one, because fat tissue converts other hormones into oestrogen, producing a continuous low-level stimulus that persists after menopause. Polycystic ovary syndrome causes infrequent ovulation and therefore infrequent progesterone exposure. Oestrogen-only hormone therapy in a woman with a uterus has the same effect, which is why progestogen is always added unless the uterus has been removed. Tamoxifen, used in breast cancer treatment, acts on the endometrium in a way that increases risk and commonly causes thickening. Diabetes, never having been pregnant, early first period, late menopause, and oestrogen-producing ovarian tumours all contribute. Lynch syndrome, an inherited condition, carries a substantially elevated lifetime risk and has implications for relatives.
Several other explanations for a thickened appearance are benign and worth knowing. Fluid within the cavity can be misread as thickening if the measurement technique does not account for it. Blood clots, retained products after pregnancy, and infection produce apparent thickening. Adhesions and previous surgery can distort the appearance. Submucosal fibroids indent the cavity and may be mistaken for a lining abnormality. In women taking tamoxifen, characteristic subendometrial cystic changes often produce a markedly thickened measurement that frequently proves benign on sampling.
Symptoms determine how the finding is approached far more than the measurement itself.
Bleeding is the key symptom, and one rule dominates this entire topic: any vaginal bleeding after menopause requires investigation. This includes light spotting, a single episode, pink or brown discharge, and bleeding a woman assumes must be from elsewhere. It is the earliest sign of endometrial cancer and the reason the disease is usually caught early. Most women with postmenopausal bleeding turn out to have a benign cause—atrophy of the vaginal and endometrial tissue is the most common—but the only way to establish that is to investigate.
Before menopause, symptoms warranting assessment include heavy or prolonged periods, bleeding between periods, bleeding after intercourse, and persistently irregular cycles, particularly in women with risk factors.
Persistent watery or blood-tinged discharge, pelvic pain, and difficulty conceiving or recurrent miscarriage are less common presentations.
A thickened lining with no symptoms in a woman who still has periods is usually of no significance. In a postmenopausal woman without bleeding, incidental thickening is a more nuanced situation, discussed below.
Evaluation follows a reasonably clear sequence.
Transvaginal ultrasound is the first-line test. The lining is measured at its thickest point in the midline sagittal plane, including both layers, with any fluid in the cavity excluded from the measurement. The examination also assesses whether thickening is uniform or focal, whether the boundary with the underlying muscle is intact, and whether there is increased blood flow to a focal lesion.
In postmenopausal women with bleeding, a well-established threshold applies. A lining measuring at or below approximately 4 millimetres makes endometrial cancer very unlikely, and in this situation further sampling is often unnecessary, though persistent or recurrent bleeding warrants investigation regardless of thickness. A measurement above this threshold prompts tissue sampling. This threshold applies specifically to postmenopausal women who are bleeding and should not be applied to other groups.
In postmenopausal women without bleeding, no agreed threshold exists, and this is genuinely uncertain territory. Incidental thickening in an asymptomatic woman has a much lower likelihood of malignancy, and the decision about whether to investigate weighs the measurement, the appearance, and the woman’s individual risk factors. Overinvestigating this group causes anxiety and unnecessary procedures, while ignoring substantial thickening in a high-risk woman is also unwise. This is a reasonable point at which to ask a clinician to explain the specific reasoning.
In premenopausal women, thickness thresholds are of limited use because of cyclical variation, and assessment focuses instead on symptoms, the appearance of the lining, and the timing of the scan within the cycle.
Saline infusion sonohysterography instils a small amount of fluid into the cavity during ultrasound, outlining its contours. It is particularly good at distinguishing a focal lesion such as a polyp from generalised thickening, which matters because focal lesions can be missed by blind sampling.
Hysteroscopy allows direct visualisation of the cavity with a fine telescope and permits targeted biopsy or removal of a lesion in the same procedure. It is the reference standard for focal abnormalities and is frequently performed in an outpatient setting.
Endometrial biopsy provides tissue for diagnosis and can often be taken in clinic with a fine suction device. Its limitation is that it samples blindly and can miss a focal lesion, so a negative biopsy in a woman with persistent bleeding does not end the investigation.
MRI is used to stage confirmed endometrial cancer, assessing depth of invasion into the muscle and involvement of the cervix and lymph nodes, rather than for initial assessment of thickening.
Important to Know
The single most important message on this topic is straightforward: any bleeding after menopause should be investigated, no matter how light or how brief. Care is typically coordinated by gynaecologists, with primary care initiating referral and oncology involved where cancer is confirmed.
That message deserves emphasis because the barrier is usually not access but assumption. Women frequently dismiss a single episode of spotting, attribute brown discharge to something else, or assume that because bleeding stopped it must have been nothing. Endometrial cancer is highly curable when caught early, and early bleeding is precisely what makes that possible. Most women investigated will have a benign explanation, and that reassurance is worth obtaining.
Equally, a thickened lining reported incidentally on a scan in a woman who still has periods and has no unusual bleeding is frequently just a normal lining measured at a particular point in the cycle. Reports describing this are recording a measurement, not raising an alarm, and the appropriate step is to discuss it with the clinician who ordered the scan rather than to interpret the number alone.
Treatment depends entirely on what is found.
Polyps causing bleeding are removed hysteroscopically, which usually resolves symptoms and provides tissue for examination. Removal is generally recommended for polyps in postmenopausal women given the higher chance of significant change within them, and for women with fertility concerns. Small asymptomatic polyps in premenopausal women may reasonably be observed, as a proportion regress spontaneously.
Hyperplasia without atypia is treated with progestogen, and the levonorgestrel-releasing intrauterine system is generally the preferred option because it delivers treatment directly to the lining with fewer systemic effects and has the best evidence for achieving regression. Treatment continues for at least six months, with repeat sampling to confirm the lining has returned to normal. Addressing contributing factors—particularly weight, where relevant—improves outcomes and reduces recurrence.
Atypical hyperplasia is managed quite differently because of the substantial risk of coexisting or subsequent cancer. Hysterectomy is usually recommended for women who have completed their families. For women wishing to preserve fertility, treatment with high-dose progestogen, usually via an intrauterine system, with close surveillance and repeated sampling, is possible in carefully selected cases, ideally with fertility specialist involvement and a plan for definitive surgery once childbearing is complete.
Endometrial cancer is treated primarily with surgery—removal of the uterus, tubes, and ovaries, with lymph node assessment guided by risk. Radiotherapy and chemotherapy are added according to stage and risk features. Molecular classification of endometrial cancer has become an important part of assessment, identifying subgroups with markedly different outlooks and treatment responses, and is increasingly used to guide whether additional treatment is needed.
Tamoxifen deserves specific mention, since many women taking it develop endometrial thickening that proves benign. Routine ultrasound surveillance of asymptomatic women on tamoxifen is generally not recommended, precisely because it detects changes that lead to unnecessary procedures. What matters instead is that any abnormal bleeding is reported and investigated promptly. Women on tamoxifen should be told this clearly at the outset.
Risk reduction is meaningful for this group of conditions. Maintaining a healthy weight has a substantial effect, since fat tissue is a continuing source of oestrogen. Combined hormonal contraception and the levonorgestrel intrauterine system both reduce endometrial cancer risk. Ensuring progestogen is included alongside oestrogen in hormone therapy for any woman with a uterus is essential. Managing diabetes and treating anovulatory cycles in polycystic ovary syndrome both help.
Lynch syndrome warrants consideration where there is a personal or family history of endometrial, colorectal, ovarian, or related cancers, particularly at young ages. Identifying it changes surveillance recommendations and has implications for relatives, and endometrial cancer tissue is now routinely screened for features suggesting it.
Care is typically coordinated by gynaecology, with gynaecological oncology, fertility, genetics, and primary care involvement as needed. Imaging, sampling, and clinical findings are interpreted alongside menopausal status, bleeding pattern, risk factors, and reproductive plans rather than in isolation.
Patient education plays an important role. Understanding that endometrial thickness means different things before and after menopause, that a measurement without symptoms is often insignificant, that postmenopausal bleeding always warrants investigation, that most causes are benign, and that a normal biopsy does not end the matter if bleeding continues all contribute to appropriate care.
Red flag symptoms include any vaginal bleeding after menopause, including a single episode of spotting or pink or brown discharge; bleeding between periods, after intercourse, or a clear change in a previously regular pattern; heavy bleeding causing dizziness, breathlessness, or a racing heart; persistent watery or blood-stained discharge; new pelvic pain or pressure; unexplained weight loss; abdominal swelling; and any abnormal bleeding in a woman taking tamoxifen or oestrogen-only hormone therapy. These warrant prompt medical evaluation.