During your period, your uterus is actively shedding its inner lining and contracting to push that tissue out through your cervix. From the outside, it looks the same as always, roughly the size and shape of an upside-down pear. But inside, a dramatic transformation is underway: the thick, blood-rich lining that built up over the previous weeks is breaking apart and leaving your body, temporarily reducing the inner lining from as thick as 16 to 18 millimeters down to a thin strip of just 1 to 4 millimeters.
How the Lining Changes Through Your Cycle
To understand what your uterus looks like on your period, it helps to know what it looked like right before. In the two weeks leading up to menstruation (the secretory phase), the endometrial lining reaches its peak thickness of about 16 to 18 millimeters. This lining is soft, spongy, and packed with tiny blood vessels called spiral arteries, all designed to support a potential pregnancy.
When pregnancy doesn’t happen, hormone levels drop, and those spiral arteries constrict and break down. The top layer of the lining, called the functional layer, detaches and sheds. On an ultrasound during menstruation, the lining appears as a thin, bright line measuring only 1 to 4 millimeters. That’s a reduction of roughly 80 to 90 percent of its peak thickness, all happening over the course of a few days.
What the Uterus Is Doing During Your Period
Your uterus isn’t just passively letting tissue fall away. The muscular wall (the myometrium) contracts rhythmically to help expel the shedding lining, and these contractions are what you feel as menstrual cramps. Ultrasound studies have captured these contractions as visible waves rippling through the uterine wall. During menstruation, these waves move from the top of the uterus down toward the cervix, essentially squeezing blood and tissue out the exit.
The uterus also changes in overall size across the menstrual cycle. Ultrasound measurements show the uterus grows slightly larger toward the end of each cycle, when the lining is at its thickest and blood flow to the organ is at its peak. Once menstruation begins and the lining sheds, the uterus gradually returns to its smaller baseline size.
What Your Cervix Looks Like
The cervix, the narrow opening at the bottom of your uterus, also shifts during your period. It opens slightly to allow menstrual blood and tissue to flow out. If you were to feel it with a finger during menstruation, it would sit relatively low in the vaginal canal and feel firmer than it does at other points in your cycle. By comparison, during ovulation the cervix moves higher, softens, and becomes harder to reach. These changes happen on a subtle scale, but they’re consistent enough that some people track cervical position as part of fertility awareness.
What the Shed Tissue Looks Like
The material that comes out during your period is a mix of blood, mucus, and endometrial tissue. Its color typically ranges from bright red to dark brown, depending on how quickly it leaves the body. Fresher blood tends to be red; blood that takes longer to exit oxidizes and turns darker. Small clots, usually smaller than a quarter, are normal and simply represent blood that pooled briefly before being expelled.
Most of the time, the lining sheds in fragments rather than as a single piece. In rare cases, the entire lining comes out intact in what’s called a decidual cast. This looks like a fleshy, pinkish-red piece of tissue shaped like the inside of the uterus, roughly resembling a triangle or light bulb. It’s larger than a typical clot and has a more structured, almost meat-like texture. While startling to see, a decidual cast is not usually a sign of a serious problem.
How Much Fluid Your Uterus Releases
The total volume of menstrual fluid over a full period averages about 80 to 87 milliliters, which is less than half a cup. The normal range is wide, though, spanning from as little as 15 milliliters to over 160. People who have given birth tend to have slightly heavier periods, with a typical volume around 93 milliliters. Anything over about 170 milliliters is generally considered heavier than normal and may be worth discussing with a provider, especially if it’s accompanied by fatigue or other symptoms of iron deficiency.
Keep in mind that menstrual fluid isn’t pure blood. It’s a combination of blood, tissue fragments, cervical mucus, and vaginal secretions, which is why it often looks and feels different from blood you’d see from a cut.
What It Looks Like on Ultrasound
If you happen to have a pelvic ultrasound during your period, the image looks noticeably different from one taken mid-cycle. The endometrial lining appears as a thin, bright (echogenic) line down the center of the uterus, sometimes with small pockets of fluid or blood visible in the cavity. Mid-cycle, that same lining appears as a thicker, layered structure with a distinctive three-stripe pattern. Doctors sometimes prefer to schedule ultrasounds right after menstruation ends, when the thin lining makes it easier to spot abnormalities like polyps or fibroids that might otherwise be hidden in thicker tissue.
On ultrasound, the muscular wall of the uterus looks largely the same throughout the cycle: a uniform gray layer surrounding the central lining. But real-time imaging can sometimes capture the subtle contractions moving through the muscle, visible as slow waves traveling from the top of the uterus downward during menstruation.