Uterine polyps form when the tissue lining the uterus (the endometrium) overgrows in a localized area, creating a small, finger-like or round projection that stays attached to the uterine wall. The underlying trigger is a hormonal imbalance in how that tissue responds to estrogen and progesterone, though several other factors increase the likelihood of developing them. They’re common: among women with abnormal uterine bleeding, 20 to 40 percent have endometrial polyps, and roughly 62 percent of women who have them experience no symptoms at all.
The Hormonal Imbalance Behind Polyp Growth
Your uterine lining naturally thickens and sheds each menstrual cycle in response to estrogen and progesterone. Estrogen promotes growth, and progesterone keeps that growth in check. When this balance tips, patches of endometrial tissue can keep growing instead of shedding normally.
What makes a polyp different from the surrounding tissue is surprisingly specific. Research comparing polyp tissue to normal endometrium found that the supportive tissue (stroma) inside polyps has significantly fewer progesterone receptors than the tissue around it. This means polyp tissue is less responsive to progesterone’s restraining effect, so it continues to grow even when the rest of the lining responds normally to hormonal signals. The estrogen receptors in polyp tissue, by contrast, are only slightly reduced. The net result is tissue that gets the “grow” signal from estrogen but barely hears the “stop” signal from progesterone.
This receptor imbalance also explains why polyps don’t shed during menstruation the way normal endometrial tissue does. They’re essentially insensitive to the cyclic hormonal changes that trigger your period, which is why they persist and gradually enlarge over time.
Age Is the Strongest Risk Factor
Getting older is the single most reliable predictor of developing uterine polyps. A study examining whether conditions like high blood pressure, obesity, and diabetes independently increase polyp risk found that, after adjusting for all variables, only age remained statistically significant. The other conditions appeared to raise risk on their own in initial analysis, but their effect disappeared once age was accounted for, meaning they likely reflect the broader changes that come with aging rather than directly causing polyps.
This makes biological sense. As you approach and enter menopause, your hormone levels fluctuate more dramatically. Estrogen can remain elevated or fluctuate unpredictably even as progesterone drops, creating exactly the kind of imbalance that promotes polyp growth. Roughly 10 percent of all women have endometrial polyps found at autopsy, suggesting many go undetected throughout life.
Tamoxifen and Medication-Related Causes
Tamoxifen, a medication widely used in breast cancer treatment, is one of the clearest medication-related causes of uterine polyps. The drug works by blocking estrogen’s effects in breast tissue, but it has a paradoxical effect on the uterus: there, it acts more like estrogen, stimulating the endometrial lining to grow. In standard doses, tamoxifen is associated with endometrial thickening, polyp formation, and in rarer cases, more serious changes to uterine tissue.
The risk is most pronounced in older women. In a prevention trial, women aged 50 and older taking tamoxifen had roughly four times the risk of developing endometrial cancer compared to those on a placebo. The annual rate of uterine malignancies was 3.05 per 1,000 women on tamoxifen versus 0.76 per 1,000 on placebo. While these numbers refer specifically to cancer rather than polyps, polyp formation is part of the same spectrum of endometrial changes tamoxifen causes, and polyps are far more common than cancer in this group.
Tamoxifen also makes monitoring more complicated. It causes a thickening of tissue just beneath the surface of the endometrium, which can look like an abnormality on ultrasound even when there’s nothing concerning. This means ultrasound measurements of endometrial thickness in tamoxifen users don’t correlate well with actual problems, and additional testing is sometimes needed to get a clear picture.
Conditions That Often Coincide
Obesity, high blood pressure, and diabetes frequently appear alongside uterine polyps, though the relationship is more about shared risk than direct cause. All three conditions become more common with age, and all three involve metabolic changes that can influence hormone levels. Fat tissue, for instance, produces its own estrogen, which is one reason obesity has long been considered a polyp risk factor.
However, the research suggests these conditions may not independently cause polyps so much as travel in the same company. When age is factored out, their statistical link to polyps weakens considerably. That said, if you have several of these conditions, the cumulative hormonal environment they create may still tip the balance toward polyp development, even if no single one is the definitive trigger.
What Polyps Look Like on Imaging
Uterine polyps are most often discovered during a transvaginal ultrasound, where they show up as focal masses within the endometrial lining or as a nonspecific thickening. They can appear brighter, darker, or similar in density compared to the surrounding tissue, which is one reason they can be tricky to identify. Ultrasound picks up polyps with about 90 percent sensitivity, meaning it catches most of them, but only about 39 percent specificity, meaning it sometimes flags things that turn out not to be polyps.
Fibroids, which grow from the muscular wall of the uterus rather than the lining, typically appear as well-defined solid masses and may contain calcifications or areas of degeneration. When a fibroid grows near the inner surface of the uterus (submucosal), it can look very similar to a polyp on ultrasound, making the distinction difficult without further evaluation. A procedure called hysteroscopy, where a small camera is inserted into the uterus, provides a direct view and is considered the gold standard for confirming a polyp.
When Polyps Become Concerning
The vast majority of uterine polyps are benign. In premenopausal women, about 1.1 percent of polyps show malignant changes. That rate rises to roughly 5 percent in postmenopausal women, with the risk being higher in those who have symptoms like unexpected bleeding. Among postmenopausal women without symptoms, the malignancy rate drops to about 1.5 percent.
The practical takeaway is that polyps are more likely to warrant removal in postmenopausal women, especially those experiencing bleeding. In premenopausal women with small, asymptomatic polyps, monitoring over time is a reasonable approach since some polyps resolve on their own. Larger polyps, those causing heavy or irregular bleeding, or those found after menopause are more likely to be evaluated with a tissue sample or removed altogether.