The womb, or uterus, is a hollow, muscular organ roughly the size and shape of an upside-down pear. In its non-pregnant state, it measures about 8 cm long, 5 cm wide, and 4 cm thick, making it small enough to fit in the palm of your hand. Its outer surface is smooth and pinkish, covered by a thin, glistening membrane called the peritoneum, the same slippery tissue that lines most of your abdominal organs.
Shape, Size, and Position
The uterus sits low in the pelvis, nestled between the bladder in front and the rectum behind. In most women, it tilts slightly forward over the bladder, a position called anteverted. About one in four women have a uterus that tips backward instead (retroverted), which is a normal variation that doesn’t usually cause problems.
The wider, rounded top portion is called the fundus. The main body narrows as it moves downward, tapering into the cervix, a firm, cylindrical neck that opens into the vaginal canal. On each side near the top, the fallopian tubes branch outward like small arms reaching toward the ovaries. Several pairs of ligaments anchor the whole structure in place, keeping it relatively stable while still allowing some movement as the bladder and bowels fill and empty throughout the day.
What the Walls Are Made Of
If you could slice the uterus open, you’d see three distinct layers. The outermost layer is the serosa, that thin, smooth peritoneal covering visible from the outside. Beneath it lies the myometrium, a thick layer of muscle that makes up the bulk of the uterine wall. This muscle tissue is what generates contractions during labor and menstrual cramps during your period. On ultrasound, it appears as a uniform, slightly bright texture, and when the wall thickness is symmetrical on both sides, that’s considered healthy.
The innermost layer is the endometrium, a soft, velvety lining rich in blood vessels and glands. This is the layer that changes dramatically throughout the menstrual cycle and the layer that sheds during your period. Between the endometrium and the main muscle, there’s a thin, darker band called the junctional zone, which acts as a boundary between the two.
How the Lining Changes Each Month
The endometrium is the most visually dynamic part of the uterus. In the first half of your cycle, after your period ends, it rebuilds under the influence of estrogen, thickening to about 12 to 13 millimeters (roughly half the width of a pencil) by the time you ovulate. After ovulation, it continues to grow and becomes spongier, preparing to receive a fertilized egg. Just before your period, it reaches its peak thickness of about 16 to 18 millimeters.
If pregnancy doesn’t occur, the top layers of the endometrium break down and shed as menstrual blood, and the cycle starts over. This constant building and shedding means the inside of the uterus looks different depending on when in your cycle you’re looking at it. On ultrasound during the first few days after a period, the lining appears as a thin, faint line. By mid-cycle, it shows up as a thicker, brighter stripe with a characteristic three-layered pattern.
How It Looks on Ultrasound
Most people will never see their uterus directly, but many will see it on an ultrasound screen during a routine exam or pregnancy scan. On ultrasound, a healthy uterus appears as a pear-shaped structure with smooth, regular outer contours. The muscular wall shows a uniform gray tone throughout. The endometrial lining appears as a brighter central stripe, and the junctional zone shows up as a slightly darker band just underneath it.
Radiologists look at several features when reading a uterine ultrasound: whether the outer surface is smooth or irregular, whether the walls are symmetrical in thickness, and whether the muscle tissue has a consistent texture or contains any unusual bright or dark spots. Irregularities in any of these features can point to conditions like fibroids or adenomyosis.
How Pregnancy Transforms the Uterus
The uterus undergoes one of the most dramatic physical changes of any organ in the human body during pregnancy. It starts at roughly 70 grams, about the weight of a small egg, and grows to approximately 1,100 grams by full term, a nearly 16-fold increase. Its internal volume expands from about 10 milliliters to 5 liters, enough to hold the baby, placenta, and amniotic fluid.
By the end of pregnancy, the top of the uterus (the fundus) reaches all the way up to the rib cage, having risen out of the pelvis entirely. The muscular wall stretches and thins to accommodate this growth, while new muscle fibers and blood vessels develop to support the expanding organ. The shape shifts from a compact pear to something more like a large, rounded balloon.
After delivery, the uterus begins shrinking back toward its original size through a process called involution. The most rapid shrinking happens in the first few days postpartum. Within about six weeks, the uterus has typically returned close to its pre-pregnancy dimensions, though it may remain slightly larger than it was before a first pregnancy.
Variations That Are Normal
Not every uterus looks the same. Size varies naturally based on age, whether someone has been pregnant before, and individual genetics. A uterus that has carried pregnancies tends to be somewhat larger and heavier than one that hasn’t. After menopause, the uterus gradually shrinks as hormone levels decline, and the endometrial lining becomes much thinner.
Some women are born with uterine shape variations. A bicornuate uterus has two horn-like projections at the top, giving it a heart shape instead of a pear shape. A septate uterus has a wall of tissue dividing part of the interior cavity. These structural differences are present from birth and are often discovered incidentally during imaging for something else. Most don’t cause symptoms, though some can affect fertility or pregnancy outcomes.