What Does It Mean When Endometrial Cells Are Present?

Endometrial cells found on a Pap test are, for most premenopausal women, a normal byproduct of the menstrual cycle and carry little clinical significance on their own. Their meaning changes substantially once a woman reaches her mid-40s or is postmenopausal, when the same finding can signal underlying endometrial disease that warrants further investigation. The distinction between a reassuring incidental finding and a red flag comes down largely to age, menopausal status, and whether the cells look normal or atypical under the microscope.

Why Endometrial Cells Show Up on a Pap Test

A Pap test is designed to sample cervical cells, but the collection brush sometimes picks up cells from higher in the reproductive tract, including the endometrium (the lining of the uterus). During and just after menstruation, the endometrium actively breaks down and sheds. The process is driven by a drop in progesterone, which triggers a cascade of enzymes that degrade the tissue so it can be expelled.

Because of this normal shedding, endometrial cells are commonly found in Pap tests collected during the first half of the menstrual cycle. One large study found that no asymptomatic premenopausal women shedding normal-appearing endometrial cells had significant endometrial pathology, regardless of whether the cells appeared before or after day 12 of their cycle.¹ In other words, if you are still having regular periods and the cells look benign, there is very little reason for alarm. The timing of your Pap relative to your period is one of the most common explanations for the finding.

The identification of these cells is not always straightforward. Recognizing endometrial cells on a slide requires experience, and accuracy improves when the cytologist knows where you are in your cycle and whether you use any form of hormonal contraception.²

The Bethesda System and the Age-45 Threshold

Pathologists report Pap test findings using a standardized framework called the Bethesda System. The 2014 edition of those guidelines specifically recommends that labs flag benign-appearing endometrial cells when they show up in women aged 45 and older.³ Before that update, the threshold was 40, and that lower cutoff generated a wave of follow-up biopsies in younger women that turned up very little disease. Research showed the recommendation led to what amounted to unwarranted surveillance in many 40-to-44-year-olds.⁴

The age cutoff matters because the risk of endometrial cancer rises with age and proximity to menopause. Raising the threshold to 45 was an attempt to balance early detection against unnecessary procedures. That said, the threshold is a guideline, not a biological cliff. A 43-year-old with abnormal bleeding and endometrial cells on her Pap may still be investigated, while a 46-year-old who had her Pap on day 6 of a regular cycle may not need immediate workup. Clinical context always modifies how the lab report is interpreted.

What the Risk Actually Looks Like by Age Group

The numbers help put the finding in perspective. Among premenopausal women aged 40 to 44 who had endometrial cells on their Pap, about 4 percent were found to have abnormal biopsy results on follow-up, and most of those abnormalities were low-grade. No high-grade cancers were detected in that group. In premenopausal women 45 and older, the picture shifted: roughly 8 percent had biopsy abnormalities, and about 3.4 percent had cancer, some of which were high-grade. Strikingly, several of those cancer patients had no symptoms at all.⁵

In postmenopausal women, the stakes climb further. One study found that when normal-appearing endometrial cells were present on a cervical smear in asymptomatic postmenopausal women, the rate of precancerous or malignant uterine disease was about 6.5 percent, compared to just 0.2 percent in smears that did not contain those cells. That translates to roughly a 40-fold increase in relative risk.⁶ Another study of women 40 and older found that being postmenopausal and being 50 or older were both independently associated with discovering precancerous or cancerous endometrial changes when endometrial cells appeared on a Pap test.⁷

These numbers deserve some context. A 6.5 percent risk of significant disease sounds worrying, and it is high enough to justify investigation. But it also means that the large majority of postmenopausal women with endometrial cells on a Pap will turn out to be fine. The finding is a reason for follow-up, not a diagnosis.

Benign Cells Versus Atypical Cells

The language on your Pap report matters. “Benign-appearing endometrial cells” is a very different finding from “atypical endometrial cells.” The Bethesda System subdivides atypical endometrial cells into two categories: “not otherwise specified” (NOS), which is the more common designation, and “favor neoplasia,” which is less common but carries meaningfully higher risk.

Among patients with atypical endometrial cells, research found that those classified as “favor neoplasia” had endometrial cancer at a rate of about 27 percent, compared to roughly 10 percent for the NOS group. The odds of carcinoma were more than three times higher in the “favor neoplasia” category.⁸ If your report says “atypical endometrial cells,” your doctor will almost certainly recommend further evaluation regardless of your age, and the specific subcategory helps determine how urgently.

How Hormones and Devices Affect the Finding

Several common medications and devices can make endometrial cells more likely to appear on a Pap test, independent of any disease process.

Hormone replacement therapy (HRT) in postmenopausal women significantly increases the prevalence of benign endometrial cells on Pap smears. One study found roughly a 56 percent higher rate of these cells in HRT users compared to non-users.⁹ This makes sense: exogenous estrogen stimulates the endometrial lining to grow and can provoke shedding, so cells have more opportunities to make their way down to the cervix. The practical implication is that if you are on HRT and your Pap shows benign endometrial cells, your doctor should factor in your medication before leaping to a biopsy, although follow-up may still be warranted depending on other risk factors.

Intrauterine devices also change the picture. Copper IUDs in particular are associated with a higher prevalence of bland endometrial cells on Pap tests.¹⁰ The copper device sits inside the uterus and causes a low-grade inflammatory response that can promote shedding. Hormonal IUDs releasing levonorgestrel, by contrast, tend to thin the endometrial lining over time, resulting in atrophy rather than active shedding.¹¹ Progestin-only implants produce a similar thinning effect.¹² Knowing what contraception you use helps your clinician interpret an endometrial cell finding more accurately.

What Happens After the Finding

When endometrial cells on a Pap test prompt further investigation, the typical next steps are an ultrasound and sometimes an endometrial biopsy. Transvaginal ultrasound measures the thickness of the endometrial lining; a very thin lining in a postmenopausal woman is generally reassuring. In one study, ultrasound using a thickness cutoff of 4 mm had a sensitivity of 82 percent for detecting endometrial disease, and all six patients with endometrial carcinoma had lining thicknesses above 12 mm.¹³

If the ultrasound shows a thickened lining, or if symptoms like abnormal bleeding are present, an in-office endometrial biopsy using a thin catheter (often called a Pipelle) is the standard next step. The same study found that the Pipelle detected all cases of endometrial carcinoma, though its overall sensitivity for endometrial disease more broadly was lower, around 45 percent. That gap means a negative Pipelle result does not always rule out non-cancerous conditions like polyps or hyperplasia, and a hysteroscopy with directed biopsy may follow if clinical suspicion remains high.

For premenopausal women under 45 with benign-appearing endometrial cells and no symptoms, most guidelines suggest no automatic workup. The finding is noted, and the patient is monitored at routine intervals. But if you have abnormal bleeding, a strong family history of endometrial or colon cancer, or other risk factors like obesity or polycystic ovary syndrome, your doctor may reasonably choose to investigate even if the cells themselves look normal.

Limitations of the Pap Test for Detecting Endometrial Cancer

It is worth knowing that the Pap test was never designed as an endometrial cancer screen. It was designed for cervical cancer. When endometrial cells do appear, they can sometimes offer an early clue, but overall the Pap test catches endometrial cancer in roughly 45 percent of patients who have it. Sensitivity is somewhat higher for more advanced cancers and for non-endometrioid subtypes, which are generally the more aggressive forms.¹⁴ That means more than half of endometrial cancers would not be flagged by a Pap test. Relying on it as your only screen for endometrial disease is not sufficient, and any abnormal uterine bleeding should be evaluated on its own merits regardless of what your most recent Pap showed.

Endometrial Cells Outside the Uterus

Endometrial cells do not only travel downward toward the cervix. During menstruation, small amounts of menstrual fluid can flow backward through the fallopian tubes into the pelvic cavity, a process called retrograde menstruation. This means endometrial cells can be found in peritoneal fluid, the thin layer of fluid surrounding the organs in the abdomen.

Researchers have found endometrial cells in peritoneal fluid more frequently during the second half of the menstrual cycle.¹⁵ This is a common occurrence, and the finding alone does not mean a woman has endometriosis. One study found no significant difference in the frequency of endometrial tissue in peritoneal fluid between women with endometriosis and those without it.¹⁶ Another study using specific cell markers confirmed similar results: the prevalence of endometrial cells in peritoneal fluid was not higher in patients with endometriosis than in controls.¹⁷

This is a genuinely surprising finding for many people, since retrograde menstruation was long considered the primary explanation for how endometriosis develops. The evidence suggests that retrograde flow of endometrial cells is nearly universal, but endometriosis only develops in some women. The difference probably involves immune dysfunction, genetic susceptibility, or properties of the cells themselves rather than simply the presence of stray endometrial tissue. Animal research has shown that when endometrial cells do integrate into existing endometriotic lesions, they tend to contribute to the stroma and blood vessels rather than forming new glandular tissue, and the majority of integrated cells are immune cells.¹⁸

How Endometriotic Cells Evade Immune Clearance

If most women have endometrial cells floating in their pelvic cavity and most of them never develop endometriosis, the immune system must be clearing those cells efficiently in most cases. Research has started to clarify how endometriotic tissue evades that clearance when the normal process fails.

Endometriotic tissue releases tiny vesicles called exosomes that carry molecules on their surface capable of disarming nearby immune cells. These exosomes carry ligands that downregulate a key activating receptor on immune cells, reducing their ability to mount a cytotoxic attack. They also carry molecules that trigger programmed cell death in activated immune cells. The net effect is an immunosuppressive zone around the endometriotic lesion, sometimes described as a “protective shield,” that prevents the immune system from clearing the displaced tissue.¹⁹ Understanding this mechanism may eventually lead to therapies that strip away that shield, but for now it mainly helps explain why endometrial cells that should be harmless become persistent implants in some women.

Adenomyosis and Inward Migration

Endometrial cells do not only migrate outward. In adenomyosis, endometrial tissue grows into the muscular wall of the uterus itself. The prevailing theory is that this happens through invagination: repeated micro-injuries to the junction between the endometrium and the underlying muscle layer allow the deepest layer of the endometrium to push inward over time.²⁰ Adenomyosis can cause heavy, painful periods and is increasingly recognized as a contributor to infertility. It is diagnosed by imaging (MRI or ultrasound) rather than by Pap test, and finding endometrial cells on a cervical smear does not indicate adenomyosis. But for anyone researching what it means for endometrial cells to appear where they should not, adenomyosis is a related condition worth understanding.

Emerging Non-Invasive Detection Methods

The future of detecting endometrial disease may move beyond the Pap test entirely. Researchers are exploring liquid biopsy approaches that screen blood or even urine for molecular signatures of endometrial cancer. Urine-derived exosomes, for example, carry small RNA molecules whose expression patterns differ between women with endometrial cancer and those without it. Early work has identified specific microRNA candidates that could serve as diagnostic biomarkers, potentially offering a completely non-invasive way to screen for the disease.²¹ These technologies are still in the research phase and not yet ready for routine clinical use, but they represent a direction that could eventually replace the somewhat accidental way that endometrial cancer is currently discovered on cervical cytology.²²