The uterus is a hollow, muscular organ in the pelvis where a fertilized egg implants and a fetus develops during pregnancy. In its non-pregnant state, it’s roughly the size of a small pear, measuring about 7.6 by 4.5 by 3 centimeters and weighing around 70 grams. Despite its modest size, the uterus is one of the most dynamic organs in the body, capable of expanding to 500 times its original volume during pregnancy.
Parts of the Uterus
The uterus has four distinct regions, arranged from top to bottom. The fundus is the broad, curved top portion where the fallopian tubes connect on either side. Below it sits the corpus (or body), which makes up the main bulk of the organ and is where a developing embryo implants. The isthmus is a narrow transitional segment connecting the body to the lowest part: the cervix, which projects downward into the vagina and serves as the gateway between the uterine cavity and the outside world.
Three Layers of the Uterine Wall
The uterine wall is built from three layers, each with a different job. The innermost layer, called the endometrium, is the lining that thickens and sheds during each menstrual cycle. It has two sublayers: a deeper, stable base layer and a surface layer that responds to hormones and rebuilds itself every month. The middle layer, the myometrium, is a thick band of smooth muscle. This is what generates the powerful contractions during labor that push a baby through the birth canal. The outermost layer, the perimetrium, is a thin sheet of cells that forms a protective coating over the outside of the organ.
How the Lining Changes Each Month
The endometrium goes through a repeating cycle driven by two key hormones. During the first half of the menstrual cycle (the proliferative phase), estrogen causes the lining and its blood vessels to grow rapidly, building up a thick, nutrient-rich layer. After ovulation, progesterone takes over during the secretory phase, transforming the lining into an environment that can support a fertilized egg. The lining develops specialized glands and coiled blood vessels called spiral arteries that are ready to nourish an embryo.
If no embryo implants, progesterone levels drop, the spiral arteries constrict, and the surface layer of the endometrium breaks down and sheds. That shedding is a menstrual period. Once the period ends, the process starts over from the stable base layer, which remains intact throughout.
How the Uterus Stays in Place
The uterus isn’t rigidly fixed inside the pelvis. It’s held in position by a network of ligaments and the muscles of the pelvic floor. Two sets of ligaments do the heaviest lifting: the cardinal ligaments, which extend from the sides of the cervix to the pelvic walls, and the uterosacral ligaments, which anchor the cervix and upper vagina to the front of the sacrum (the bone at the base of the spine). Together, these hold the uterus and upper vagina in their proper position over the pelvic floor muscles.
In most people, the uterus tilts slightly forward over the bladder, a position called anteverted. About 16 to 18 percent of people have a retroverted uterus, meaning it tilts backward toward the spine instead. A retroverted uterus is typically a normal anatomical variation, not a medical problem, though it’s somewhat more common in people with pelvic floor issues.
Blood Supply
The uterus receives most of its blood from the uterine arteries, which branch off from larger vessels called the internal iliac arteries on each side of the pelvis. Once the uterine artery reaches the muscular wall, it fans out into progressively smaller branches: arcuate arteries that curve along the outer muscle, radial arteries that dive inward, and finally spiral arteries that supply the endometrial lining. The ovarian arteries also contribute some blood flow, connecting to the uterus through the fallopian tube region. This dual blood supply ensures the organ gets consistent oxygen, especially during the increased demands of pregnancy.
The Uterus During Pregnancy
Pregnancy triggers one of the most dramatic physical transformations of any organ in the body. The uterus grows from about 70 grams to roughly 1,100 grams, and its internal volume expands from 10 milliliters to about 5 liters. That’s a roughly 500-fold increase in capacity. This expansion is driven by both the stretching and growth of existing muscle cells in the myometrium and the development of new ones. The organ shifts upward out of the pelvis as it grows, eventually reaching the ribcage by the third trimester. After delivery, it contracts back toward its original size over the course of several weeks, a process called involution.
Common Uterine Conditions
Several conditions can affect the uterus, and many are quite common. Fibroids are noncancerous growths in the muscular wall that can range from tiny and symptom-free to large enough to cause heavy bleeding, pelvic pressure, or fertility problems. They’re the most common type of pelvic tumor and affect a significant portion of people with uteruses by age 50.
Endometriosis, which affects roughly 10 percent of reproductive-age women worldwide (about 190 million people according to the World Health Organization), occurs when tissue similar to the endometrial lining grows outside the uterus, often on the ovaries, fallopian tubes, or pelvic lining. It can cause severe pain, heavy periods, and difficulty getting pregnant. Adenomyosis is a related condition where endometrial tissue grows into the muscular wall of the uterus itself, often causing an enlarged uterus, heavy bleeding, and painful cramps.
Other conditions include polyps (small overgrowths of the endometrial lining), uterine prolapse (when weakened pelvic floor support allows the uterus to drop toward or into the vaginal canal), and less commonly, uterine cancer, which usually signals itself early through abnormal bleeding.
How the Uterus Is Examined
Ultrasound is the most common first step for evaluating the uterus. A transvaginal ultrasound, where a small probe is placed in the vagina, gives a clearer view of the endometrial lining and muscular wall than an abdominal scan. It can identify fibroids, polyps, and unusual thickening of the lining.
When more detail is needed, MRI provides the sharpest picture. It can distinguish between the different layers of the uterine wall and evaluate nearby structures like the ovaries, fallopian tubes, and pelvic lymph nodes, all without radiation exposure. For fertility evaluations, a hysterosalpingogram uses dye injected into the uterine cavity and X-ray imaging to check the shape of the cavity and whether the fallopian tubes are open. A similar test called a hysterosonogram fills the cavity with saline instead of dye and uses ultrasound to look for polyps or fibroids inside the lining.
CT scans are less useful for the uterus specifically because they don’t differentiate soft tissue well, though they can spot large masses. PET-CT scans are reserved mainly for staging gynecological cancers and checking for distant spread.