The human uterus, or womb, is a small, pear-shaped organ about 3 inches tall, 2 inches wide, and 1 inch thick, weighing roughly 1 ounce in its non-pregnant state. From the outside, it looks like a smooth, pale-pink muscular pouch sitting deep in the pelvis. Its size, color, and internal appearance change dramatically depending on your age, menstrual cycle phase, and whether you’re pregnant.
Shape and Size at a Glance
Picture an upside-down pear, slightly flattened front to back. That’s a reasonable stand-in for the uterus. The wider, rounded top is called the fundus. Just below it is the body, which is the largest section and where a fertilized egg typically implants. Tapering down from the body is the cervix, a narrow, cylinder-like passage that connects the uterus to the vagina. If you’ve ever seen a diagram that looks like a small triangle with two tubes branching off the top corners, those tubes are the fallopian tubes, and they attach near the fundus on either side.
Most uteruses tilt slightly forward toward the bladder, a position called anteverted. Some tilt backward toward the spine instead (retroverted). Both are normal variations that don’t usually cause problems, and they can shift over time, especially after pregnancy.
What the Outside Surface Looks Like
The outermost layer of the uterus, called the serosa, is a thin, glistening membrane made of loose connective tissue. During surgery or in anatomical photographs, it appears smooth and slightly shiny, similar to the lining you’d see on other abdominal organs. The color is typically a pale pinkish-tan, though it can look slightly more reddish in areas with greater blood supply. Beneath this outer coating sits the muscular wall, which makes up the bulk of the organ’s thickness and gives the uterus its firm, dense feel.
What the Inside Looks Like
The interior tells a different story than the smooth exterior. When doctors look inside a healthy uterus with a tiny camera (a procedure called hysteroscopy), they see a triangular cavity with smooth, pinkish walls and a flat, even surface. Near the top two corners of this triangle sit small, dark openings, one on each side, where the fallopian tubes connect. In a healthy uterus, the lining appears uniform and non-vascular at its surface level.
The lining itself, called the endometrium, is the tissue that thickens each month in preparation for pregnancy and sheds during a period. Its appearance shifts noticeably throughout the menstrual cycle. On ultrasound, the changes are easy to measure: during menstruation, the lining shows up as a thin, bright line just 1 to 4 millimeters thick. During the first half of the cycle, it develops a distinctive striped, three-layered look and grows to about 12 to 13 millimeters by ovulation. In the second half, it thickens further to 16 to 18 millimeters and becomes denser and more uniformly bright on the screen.
To your eye, the endometrium in its thickest phase would look like a soft, spongy, reddish tissue packed with tiny glands and blood vessels. When it sheds during a period, it breaks down into the mix of blood and tissue fragments that make up menstrual flow.
How Pregnancy Transforms the Uterus
The uterus undergoes one of the most dramatic size changes of any organ in the body during pregnancy. It starts at roughly the size of a clenched fist and grows continuously over 40 weeks. By week 18, it’s about the size of a cantaloupe, enough to create a visible bump. By week 25, it’s comparable to a soccer ball. Just two weeks later, around week 27, it has expanded to the size of a basketball.
This growth isn’t just stretching. The muscular wall actively adds new tissue and increases its blood supply, and the organ’s weight increases from about 1 ounce to over 2 pounds by full term. After delivery, it contracts back down over a period of roughly six weeks, though it typically remains slightly larger than it was before the first pregnancy.
How the Uterus Changes With Age
Before puberty, the uterus is very small, with a relatively long cervix compared to its body. Hormonal changes during puberty trigger it to grow into its full adult size and begin cycling. After menopause, the process reverses. As hormone levels drop, the muscular wall thins, the organ shrinks, and the endometrial lining becomes very thin. In the first five years after menopause, the lining averages about 2.3 millimeters and continues to decline slowly, dropping by a fraction of a millimeter each year before stabilizing around 1.8 millimeters. By a woman’s 70s, the average lining thickness has decreased to about 3.1 millimeters overall, compared to 5 millimeters in the early 40s. The uterus as a whole becomes noticeably smaller and paler with age.
Three Layers That Make Up the Wall
If you could slice through the uterine wall, you’d see three distinct layers stacked together. The outermost is the serosa, the thin, shiny wrapping described above. The middle layer is the thickest by far, a dense mass of smooth muscle fibers that run in multiple directions. This is the layer responsible for the powerful contractions of labor and the smaller contractions you feel as menstrual cramps. The innermost layer is the endometrium, the hormonally responsive lining that rebuilds and sheds each cycle. Together, these three layers give the uterus its unique combination of flexibility, strength, and the ability to regenerate tissue month after month for decades.