Thickening of the uterus is most often caused by excess estrogen stimulating the uterine lining without enough progesterone to balance it out. This can happen during normal phases of the menstrual cycle, but when the lining grows beyond expected ranges or thickens after menopause, it usually points to a hormonal imbalance, a structural condition like adenomyosis, or a medication side effect. Understanding which type of thickening you’re dealing with matters, because the causes, symptoms, and next steps differ significantly.
How Thick Is Normal
Your uterine lining naturally changes thickness throughout your menstrual cycle. During your period, it sheds down to about 1 to 4 millimeters. In the first half of the cycle before ovulation, it builds to around 12 to 13 millimeters. Just before your next period, it reaches its peak thickness of 16 to 18 millimeters. These fluctuations are driven by estrogen (which builds the lining) and progesterone (which stabilizes it and triggers shedding).
After menopause, the lining should stay thin. The widely used threshold is 4 millimeters: if a postmenopausal woman has vaginal bleeding and an ultrasound shows the lining is 4 mm or less, the chance of endometrial cancer is less than 1%. A measurement above 4 mm in a woman with bleeding typically prompts further testing. If you’re postmenopausal without any bleeding and a thicker-than-expected lining shows up incidentally on imaging, that alone doesn’t necessarily require further workup.
Estrogen Dominance and Endometrial Hyperplasia
The most common cause of abnormal uterine thickening is a condition called endometrial hyperplasia, where the lining overgrows because estrogen goes unopposed by progesterone. Normally, ovulation triggers progesterone production, which stops the lining from growing and eventually causes it to shed. When ovulation doesn’t happen, progesterone is never made, and the lining keeps building in response to estrogen month after month.
Several situations create this imbalance:
- Perimenopause: As you approach menopause, ovulation becomes irregular. You may still produce estrogen but skip ovulation in some cycles, leaving the lining without the progesterone signal to shed.
- Obesity: Fat tissue produces estrogen. Higher body fat means more circulating estrogen, which continuously stimulates the uterine lining.
- Polycystic ovary syndrome (PCOS): Hormonal imbalances in PCOS lead to irregular or absent ovulation, creating prolonged estrogen exposure. The World Health Organization identifies endometrial hyperplasia and endometrial cancer as recognized risks for women with PCOS specifically because of this chronic lack of ovulation.
- Estrogen-only hormone therapy: Taking estrogen without a progestin to protect the uterine lining can drive overgrowth.
Endometrial hyperplasia matters because some forms carry a risk of progressing to endometrial cancer. The type without atypical cells is lower risk and often responds to progestin treatment. The type with atypical cells has a higher chance of becoming cancerous and is managed more aggressively.
PCOS and Chronic Anovulation
PCOS deserves special attention because it’s one of the most common hormonal conditions in women of reproductive age, and it creates the perfect setup for uterine thickening. The core problem is inappropriate hormonal signaling that raises androgen levels and disrupts the normal ovulation cycle. When you don’t ovulate regularly, you don’t produce progesterone regularly, and your uterine lining has no reason to shed on schedule.
Women with PCOS may go months without a period. During that time, estrogen continues stimulating the lining unchecked. This prolonged exposure is why PCOS raises the risk of both endometrial hyperplasia and endometrial cancer. Managing PCOS with hormonal contraceptives or cyclic progestin helps protect the lining by ensuring it sheds regularly.
Adenomyosis: Thickening of the Uterine Wall
Not all uterine thickening involves the lining. Adenomyosis is a condition where tissue from the uterine lining grows into the muscular wall of the uterus (the myometrium), causing the wall itself to thicken. The uterus often becomes enlarged and takes on a globular shape. This is fundamentally different from endometrial hyperplasia, which affects only the inner lining.
The leading theory is that some injury or disruption to the boundary between the lining and the muscle allows lining tissue to push into the muscular layer, where it forms lesions. Heavy menstrual bleeding is one of the hallmark symptoms, caused by the increased surface area and blood supply that the displaced tissue creates. The severity of bleeding correlates with how deeply the tissue has invaded the muscle wall.
On ultrasound, adenomyosis can look like shadowy areas within the muscle, with a blurred boundary between the inner and outer layers of the uterus. Cystic changes may be visible. MRI provides a clearer picture, showing a widened zone between the lining and the muscle along with swelling and small cysts. Adenomyosis is benign but can cause significant pain and heavy periods.
Medications That Thicken the Uterus
Tamoxifen, commonly prescribed to treat or prevent breast cancer, is one of the best-known medication causes of uterine thickening. It works by blocking estrogen in breast tissue, but it has a weak estrogen-like effect on the uterus. This means it can stimulate the uterine lining even while fighting cancer elsewhere in the body.
The risk is meaningful. Women taking tamoxifen have roughly two to three times the risk of developing endometrial cancer compared to women not taking it. In women over 50, that risk is even higher. One large study found that women on tamoxifen developed endometrial cancer at a rate of about 1.6 per 1,000 patient-years, compared to 0.2 per 1,000 in the placebo group. Tamoxifen also causes a type of tissue growth beneath the surface lining that makes ultrasound measurements unreliable. The lining may appear thick on imaging even when no abnormal cells are present, which complicates monitoring.
Because of this, routine ultrasound screening isn’t recommended for tamoxifen users without symptoms. Instead, any new vaginal bleeding while taking the medication should be evaluated promptly.
Polyps and Fibroids
Uterine polyps are small growths that project from the lining into the uterine cavity. They can make the lining appear thicker on ultrasound and cause irregular bleeding, spotting between periods, or heavier-than-usual periods. Polyps are usually benign but occasionally contain precancerous or cancerous cells, particularly in postmenopausal women.
Fibroids, which are noncancerous growths in the muscular wall, can also contribute to the appearance of uterine thickening on imaging. Submucosal fibroids (those that bulge into the uterine cavity) are particularly likely to distort the lining and cause heavy bleeding. Both polyps and fibroids are extremely common, and many women have them without any symptoms at all.
How Thickening Is Evaluated
A standard transvaginal ultrasound is typically the first imaging step. It measures the lining’s thickness and can detect obvious masses or an enlarged uterus. However, it has limits. It can tell you the lining is thick but can’t always show why.
When more detail is needed, a sonohysterogram (also called saline infusion sonography) offers a closer look. During this test, sterile saline is injected into the uterine cavity to expand it, which outlines the lining and makes polyps, fibroids, or other masses much easier to see and measure. This test is particularly useful for pinpointing the location of growths, which matters for treatment planning. It’s also commonly used when investigating infertility or recurrent miscarriage.
If imaging raises concerns about abnormal cells, a tissue sample (biopsy) of the lining is the definitive next step. This is the only way to determine whether thickened tissue is benign, precancerous, or cancerous.
Common Symptoms to Watch For
The most frequent sign of abnormal uterine thickening is unusual bleeding. This can look like periods that are heavier or longer than normal, bleeding between periods, or any vaginal bleeding after menopause. Some women experience cycles shorter than 21 days or notice that their periods have become unpredictable after years of regularity.
Adenomyosis often adds pelvic pain and cramping to the picture, particularly during menstruation. The enlarged uterus may cause a feeling of pressure or fullness in the lower abdomen. Endometrial hyperplasia and polyps, on the other hand, tend to announce themselves primarily through bleeding changes rather than pain. In some cases, thickening produces no symptoms at all and is discovered incidentally during imaging for another reason.