The uterus is a hollow, muscular organ roughly the size of a small lemon or your closed fist. It sits in the pelvis between the bladder and the rectum, and its shape is often compared to an upside-down pear: wider and rounded at the top, narrowing as it tapers down toward the cervix. In a non-pregnant state, it measures about 3 inches long, 2 inches wide, and 1 inch thick, weighing around 1 ounce (28 grams).
Overall Shape and Parts
The wide, dome-shaped top of the uterus is called the fundus. This is where the fallopian tubes connect on each side, creating two small “horns” at the upper corners. Below the fundus is the body (or corpus), which makes up most of the organ and contains the central cavity where a pregnancy develops. The body gradually narrows into a short, slightly constricted section called the isthmus, which then transitions into the cervix, the cylindrical lower portion that opens into the vagina.
From the outside, the uterus has a smooth, pale pink surface. Its walls are thick and firm to the touch because they’re made almost entirely of muscle. If you were to slice the uterus open and look at a cross-section, you’d see three distinct layers. The outermost layer is a thin, slippery membrane that covers the surface. The middle layer is a thick wall of smooth muscle that makes up the bulk of the organ; this is the tissue that contracts during labor and menstrual cramps. The innermost layer is the lining, a soft, blood-rich tissue that thickens and sheds each month during a menstrual period.
How It Sits in the Pelvis
Most people are born with an anteverted uterus, meaning it tilts forward toward the bladder. If you imagine the uterus from the side, it leans slightly forward rather than sitting perfectly upright. Some people have a retroverted uterus, which tilts backward toward the spine instead. Both positions are normal variations and don’t typically cause problems. Ligaments and muscles in the pelvic floor hold the uterus in place, though its exact angle can shift slightly depending on how full the bladder or rectum is.
How the Lining Changes Throughout the Month
The inner lining is not static. It transforms dramatically over the course of each menstrual cycle. In the first half of the cycle, before ovulation, the lining builds up to about 12 to 13 millimeters thick (roughly half an inch). After ovulation, it continues to thicken and becomes spongier, reaching 16 to 18 millimeters just before a period begins. If pregnancy doesn’t occur, the lining breaks down and sheds. On an ultrasound, the lining appears as a bright white stripe running through the center of the uterus, and its thickness is one of the things a doctor measures to assess reproductive health.
What It Looks Like on Imaging
Most people first “see” their uterus during a pelvic ultrasound. On the screen, it appears as a gray, pear-shaped structure with a bright white line down the middle representing the lining. The muscular wall shows up as a uniform medium gray. On an MRI, the layers are even more distinct: the lining appears bright white, the outer muscle wall is a medium gray, and there’s a darker band between them where the inner muscle fibers are more tightly packed and contain less water. This layered look is a hallmark of a healthy uterus on imaging.
How Pregnancy Transforms the Uterus
During pregnancy, the uterus undergoes one of the most dramatic size changes of any organ in the body. It grows from its usual pear size to a structure large enough to hold a full-term baby, placenta, and amniotic fluid. By 40 weeks, it measures roughly 30 centimeters tall, 23 centimeters wide, and 20 centimeters deep. That’s about five times its normal dimensions in every direction.
The muscle tissue itself grows during the first 20 weeks, and the uterus increases in weight from about 50 grams to around 1,000 grams (over 2 pounds). After that midpoint, the organ doesn’t get heavier but continues to stretch to accommodate the growing baby. After delivery, it gradually contracts back toward its pre-pregnancy size over a period of several weeks, though it often remains slightly larger than it was before.
How the Uterus Changes After Menopause
After menopause, declining estrogen levels cause the uterus to shrink significantly. The reduction in size continues progressively with each year past menopause. Over time, the proportions actually reverse: the cervix becomes relatively larger compared to the body of the uterus, returning to a configuration similar to what it looked like before puberty. The lining becomes thin and atrophic, sometimes so thin on ultrasound that it’s hard to distinguish from the surrounding muscle wall. The muscle tissue itself gradually loses water content and becomes more fibrous, giving it a denser, more compact appearance on imaging.
Shape Variations Some People Are Born With
Not every uterus has the standard upside-down pear shape. Congenital uterine anomalies occur when the organ doesn’t form typically during fetal development. A bicornuate uterus is heart-shaped, with a deep indentation at the top creating two distinct “horns.” A septate uterus looks normal on the outside but has a wall of tissue (a partition) dividing the inner cavity. An arcuate uterus has a mild dent at the top of the cavity but is otherwise close to normal. A unicornuate uterus formed from only one side, making it smaller and banana-shaped. In rare cases, a uterus didelphys occurs, meaning two completely separate uterine bodies develop side by side. Some people are born without a uterus entirely. Many of these variations are discovered incidentally during imaging and may never cause symptoms, though some can affect pregnancy outcomes.