The inside of the womb, or uterus, looks different depending on whether it’s empty, in the middle of a menstrual cycle, or housing a growing pregnancy. In its non-pregnant state, the uterine cavity is a small, flattened, triangular space lined with soft, pink-to-red tissue rich in blood vessels. It’s surprisingly compact, roughly the size and shape of an upside-down pear, with an average length of about 7 centimeters in adults of reproductive age.
The Three Layers of the Uterine Wall
If you could slice through the wall of the uterus, you’d see three distinct layers stacked together. The outermost layer, called the perimetrium, is a thin protective covering. Beneath that sits the myometrium, a thick band of smooth muscle that makes up most of the uterine wall. This muscular layer is what contracts during labor and what expands dramatically during pregnancy. The innermost layer, the endometrium, is the one you’d actually see if you were looking into the cavity itself.
The endometrium is a soft, glandular lining packed with tiny blood vessels. Its surface has a velvety, slightly glistening texture because of the thin layer of mucus produced by its glands. The color ranges from pale pink to deep red depending on blood flow, and its thickness changes throughout the menstrual cycle.
How the Lining Changes Throughout the Month
Right after a period, the endometrium is at its thinnest, just a couple of millimeters, and appears relatively pale. Over the next two weeks, rising estrogen causes the lining to grow and thicken substantially. New blood vessels branch and coil through the tissue, and glands lengthen, giving the surface a plumper, spongier look with a deeper reddish tone.
After ovulation, progesterone takes over and the lining enters its secretory phase. The glands swell with nutrient-rich fluid, and the tissue becomes even more engorged with blood. At this point, the endometrium is at its thickest and most vascular, ready to receive a fertilized egg. If pregnancy doesn’t occur, hormone levels drop, the coiled blood vessels constrict, and the top layers of the lining break down and shed as menstrual flow. Then the cycle starts again.
The View From Inside During Pregnancy
Once an embryo implants, the interior of the uterus transforms into something entirely different. Within the first couple of weeks, an amniotic membrane forms and begins to line the inside of the cavity. This membrane eventually becomes the amniotic sac, a thin, translucent, two-layered bag that encloses the developing baby. The sac fills with amniotic fluid, a clear, pale yellow liquid that cushions and surrounds the fetus.
Early in pregnancy, the fluid is mostly produced by the mother’s body filtering through the membranes. Later, the baby contributes by swallowing and excreting the fluid. As the pregnancy progresses, tiny white flakes of vernix (the waxy coating that protects the baby’s skin) float in the fluid, making it slightly cloudy. Until about 22 to 25 weeks, the baby’s skin is thin enough that fluid can pass directly through it.
The Placenta and Umbilical Cord
Attached to one wall of the uterus is the placenta, a disc-shaped organ about the size of a dinner plate by full term. The side facing the uterine wall is a dark, meaty red, anchored into the blood-rich endometrium by a dense network of blood vessels. The side facing the baby is smoother and covered by the glistening amniotic membrane, with blood vessels visibly branching across its surface like the roots of a tree.
Running from the placenta to the baby’s belly is the umbilical cord, which looks like a thick, whitish, spiraling rope. It typically attaches at the center of the placenta. The cord has a distinctive coiled appearance because the blood vessels inside twist around each other, and the whole structure is cushioned by a slippery, jelly-like substance called Wharton’s jelly that gives it a firm, rubbery feel. At full term the cord is usually about 50 to 60 centimeters long.
How Much the Uterus Grows
The non-pregnant uterus is small. In a woman around age 40, the average length is about 72 millimeters, roughly the length of your index finger. The cavity inside is narrow and nearly flat, with its walls touching each other. By full term, the uterus stretches to fill most of the abdomen, expanding to roughly 500 times its original volume. The muscular wall thins out as it stretches, but gains enormous strength for labor contractions.
Women who have had previous pregnancies tend to have a slightly larger uterus even when not pregnant. Each pregnancy leaves the uterus a little bigger than before. On the other end of the age spectrum, the uterus shrinks after menopause, dropping to an average length of about 42 millimeters by age 80.
The Cervical Opening
At the very bottom of the uterine cavity, there’s a small funnel-shaped opening called the internal os. This is where the uterine cavity narrows into the cervical canal, which leads down to the vagina. In a non-pregnant state, this opening is tiny, just a few millimeters across. If you were looking up into the cavity from below, you’d see the narrow passage widen into the triangular space of the uterus, with the openings of the two fallopian tubes visible at the upper corners like small pinholes.
What the Womb Looks Like After Birth
After delivery, the inside of the uterus looks dramatically different from its pre-pregnancy state. The area where the placenta was attached leaves a raw, open wound on the uterine wall, about the size of a small plate, which gradually heals over the following weeks. The entire cavity is enlarged and the walls are thick with residual pregnancy tissue.
The shrinking process is rapid at first. On the first day after delivery, the uterus is still about 162 millimeters long in first-time mothers. By day 30, it’s roughly halved to about 81 millimeters, and by two months it’s close to its pre-pregnancy size at around 63 millimeters. The most dramatic changes happen in the first 10 days, when the cavity itself clears out and the tissue reorganizes. Ultrasound at that point typically shows a wide, fluid-filled space inside the uterus as the remaining material is reabsorbed or expelled. For women who’ve had multiple pregnancies, this process takes a bit longer, often extending past the typical six to eight week window.