The endometrial canal is the hollow, triangle-shaped space inside the uterus, lined by a tissue called the endometrium. You may have encountered this term on an ultrasound report or lab result, where it’s sometimes called the “uterine cavity” or referenced as the “endometrial stripe.” All three terms describe the same basic structure: the interior chamber of the uterus where a pregnancy develops or where menstrual tissue builds up and sheds each month.
Anatomy of the Endometrial Canal
On a cross-section view, the endometrial canal looks like an upside-down triangle. Its upper corners connect to the two fallopian tubes, and its lower point narrows into the cervical canal, which opens into the vagina. The canal sits within the main body of the uterus (the corpus) and is surrounded by a thick muscular wall called the myometrium.
The canal itself is not a wide-open space. In a non-pregnant state, the front and back walls of the uterus nearly touch each other, with only a thin layer of fluid and tissue between them. That’s why on an ultrasound, the canal typically appears as a bright line rather than a gap. This line is what radiologists call the “endometrial stripe,” and measuring its thickness is one of the most common ways doctors assess uterine health.
The Lining That Changes Every Month
What makes the endometrial canal clinically important is the tissue lining it. The endometrium is a dynamic layer that thickens and thins in response to hormones throughout each menstrual cycle. During menstruation, the lining is at its thinnest, appearing as a bright line just 1 to 4 mm thick on ultrasound. As estrogen rises in the first half of the cycle (the proliferative phase), the lining grows rapidly, reaching about 12 to 13 mm by ovulation. On ultrasound during this phase, the endometrium takes on a distinctive three-layered, striped appearance.
After ovulation, progesterone takes over. The lining continues to thicken to roughly 16 to 18 mm and becomes more uniform and bright on imaging. This secretory phase prepares the lining to receive a fertilized egg. If pregnancy doesn’t occur, hormone levels drop and the outer layers of the endometrium shed as a period, resetting the cycle.
How the Canal Supports Pregnancy
The endometrial canal’s primary reproductive purpose is to provide a site for embryo implantation. This process is far more complex than the embryo simply “landing” on the uterine wall. It requires a precise hormonal sequence and a brief window of receptivity, typically occurring about six to ten days after ovulation.
Progesterone drives most of the preparation. Under its influence, cells on the lining’s surface develop smooth, bulging protrusions that strip away a protective coating that normally prevents cells from sticking together. This molecular change makes the surface temporarily adhesive, allowing a developing embryo to attach. At the same time, the lining produces signaling molecules, including growth factors and immune-related proteins, that communicate directly with the embryo. Research has shown that embryos that successfully implant actively take up certain of these signals, while embryos that fail to implant do not, suggesting a genuine two-way conversation between the lining and the embryo.
This window of receptivity is narrow. If estrogen levels climb too high or the hormonal timing is off, the window closes quickly, and implantation becomes unlikely regardless of embryo quality.
Fluid in the Endometrial Canal
A small amount of fluid inside the endometrial canal is normal at certain points in the menstrual cycle. However, visible fluid accumulation on ultrasound, called endometrial cavity fluid, can sometimes signal a problem. In fertility treatment settings, over 60% of patients who develop this fluid have tubal-related infertility, often from a condition where fluid backs up from a damaged fallopian tube into the uterine cavity.
The concern with excess fluid is that it may create a physical barrier between the embryo and the uterine wall, preventing the embryo from making contact with the lining. Studies of IVF patients found that small amounts of fluid (less than 3.5 mm in depth) typically resolve on their own and don’t significantly reduce pregnancy rates. Larger fluid collections that persist tend to have a more serious impact. In one study, patients with persistent fluid above that threshold had a 0% clinical pregnancy rate compared to about 35% in patients whose fluid resolved.
Conditions That Affect the Canal
Several conditions can alter the shape, lining, or openness of the endometrial canal.
- Endometrial polyps are localized overgrowths of the lining that project into the canal like small fingers. They contain a mix of glandular tissue, connective tissue, and sometimes muscle fibers. Most are benign, but they can cause irregular bleeding and may interfere with fertility by disrupting implantation.
- Fibroids (specifically submucosal fibroids) are muscular growths that push into or distort the canal from the uterine wall. Even small fibroids in this location can change the shape of the cavity enough to affect bleeding patterns or embryo implantation.
- Asherman syndrome occurs when scar tissue forms inside the canal, creating bands of adhesion that stick the walls together. This most commonly happens after a surgical procedure inside the uterus. The severity ranges from mild (thin, filmy adhesions covering less than a third of the cavity, with lighter-than-normal periods) to severe (dense adhesions filling more than two-thirds of the cavity, with periods stopping entirely). Up to 31% of women who have surgical removal of a fibroid through the uterine cavity develop some degree of adhesions, and that figure rises to 46% after a second procedure.
How Doctors Examine the Canal
The most common way to evaluate the endometrial canal is transvaginal ultrasound, which measures the endometrial stripe thickness and checks for irregularities like polyps, fluid, or unusual thickening. This is a quick, non-invasive test that provides a reliable first look at the canal.
When ultrasound raises questions, a hysteroscopy offers a direct view. During this procedure, a thin scope (about 5 mm in diameter) is passed through the cervix into the canal. A gas or fluid gently expands the cavity so the doctor can inspect the lining in detail. Hysteroscopy can identify polyps, fibroids, scar tissue, and areas suspicious for hyperplasia or cancer. It also allows for targeted biopsies of specific areas rather than blind sampling, which makes it more accurate for diagnosing small or focal lesions.
Endometrial Thickness After Menopause
After menopause, the endometrial canal narrows and the lining thins significantly due to the drop in estrogen. On ultrasound, the endometrial stripe in a postmenopausal woman is expected to be thin, uniform, and continuous. The widely referenced threshold is 4 mm or less, based on guidelines from the American College of Obstetricians and Gynecologists. A lining thicker than 4 mm in a postmenopausal woman is considered a risk factor worth investigating, particularly if there is any bleeding.
For postmenopausal women without symptoms, recent research suggests the optimal screening cutoff for detecting endometrial problems may be closer to 5.65 mm. Canadian guidelines recommend further investigation when thickness exceeds 11 mm or when ultrasound shows other concerning features like increased blood flow, an uneven texture, or fluid containing particles. Factors like obesity, diabetes, high blood pressure, and use of certain medications (such as tamoxifen for breast cancer) all increase the likelihood of abnormal endometrial thickening after menopause.