What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Womb Diagram: Uterus Anatomy and Comparative Guide

Name: Womb Diagram: Uterus Anatomy and Comparative Guide
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The uterus is a hollow, muscular organ that receives the embryo and supports gestation, and its basic wall is built from three layers: an inner endometrium, a middle myometrium, and an outer perimetrium. Its shape, the length of its horns, and the structure of its cervix vary widely across species, from the single-chambered human uterus to the long-horned bicornuate uterus of the dog, cat, and sow.

A womb diagram is most useful when it separates three things that are easy to blur together: the layers of the uterine wall, the arterial supply that keeps those layers alive, and the gross shape that differs between species. This guide builds all three, then compares them in a single table.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

The Three Layers of the Uterine Wall

Every mammalian uterus, regardless of shape, is a tube of smooth muscle lined by glandular mucosa and wrapped in a serosal covering. The three named layers are the endometrium, the myometrium, and the perimetrium.

Endometrium

The endometrium is the inner mucosal lining. It sits directly against the uterine lumen and is the layer that changes most across the reproductive cycle. In species with a menstrual cycle, including humans, the endometrium thickens and is shed. In most domestic species, the endometrium is remodeled rather than shed.

The endometrium has two functional zones. The stratum functionalis is the superficial portion that responds to ovarian steroids and supports early embryo nutrition. The stratum basalis is the deeper portion that regenerates the functionalis and houses the base of the endometrial glands. The glands extend from the surface epithelium down toward the myometrium and secrete the fluid that nourishes the preimplantation embryo.

Endometrial structure is not uniform even within one uterus. In a study of non-pregnant impala, the height of the luminal epithelium was greater in the right uterine horn than the left in five of six specimens, and gland epithelium height was greater on the right side in four of those specimens [1]. That finding is a useful reminder that the two horns of a bicornuate uterus are not always mirror images at the microscopic level.

The maternal-fetal interface adds another layer of specialization once pregnancy begins. Two fibrinoid bands, Nitabuch's layer between the basal decidua and cytotrophoblasts, and Rohr's fibrinoid beneath the cytotrophoblast layer, help limit trophoblastic invasion and regulate maternal blood flow at the placental attachment site [2]. These structures belong to the pregnant uterus, but they form in the tissue that began as endometrium.

Myometrium

The myometrium is the thick smooth muscle layer. It is the engine of the uterus. It generates the contractions of estrus and parturition, and it provides the tone that keeps the organ compact when it is not pregnant.

The myometrium is conventionally described in two sublayers. The inner circular layer runs around the tube. The outer longitudinal layer runs along its length. The two orientations let the uterus shorten and narrow the lumen at the same time, which is how it expels contents.

Myometrial composition changes with reproductive history. In a morphometric study of Beagles, the absolute volume of the uterine wall did not differ between nulliparous and multiparous dogs, but the volume of blood vessels was significantly higher and the relative amount of myometrial connective tissue was significantly lower in the multiparous group [3]. The same study found that myometrial arteries in multiparous dogs had thickening of the intima with thinning of the smooth muscle layer, and that the elastica interna appeared disintegrating and highly folded [3]. These are structural fingerprints of repeated pregnancy, not disease.

Perimetrium

The perimetrium is the outer serous coat. It is a thin layer of connective tissue covered by mesothelium, and it is continuous with the broad ligament that suspends the uterus in the abdomen. The perimetrium is the smooth, glistening surface a surgeon sees first, and it is the layer that carries the surface vessels visible during laparoscopy.

Blood Supply: Uterine Artery and Ovarian Anastomosis

The uterus has a dual arterial supply, and the two sources meet.

The uterine artery is the principal supply. It arises as a branch of the internal iliac artery and travels in the broad ligament toward the uterus, where it divides into ascending and descending branches. The ascending branch runs along the lateral border of the uterus toward the uterine horn and the ovary.

The ovarian artery arises from the aorta (or, in some species, from the ovarian branch of the aorta) and reaches the ovary and the cranial tip of the uterine horn through the mesovarium. Near the tip of the horn, the ovarian artery anastomoses with the ascending branch of the uterine artery. That anastomosis is the reason the uterus can survive on either supply alone in many species, and it is the reason imaging of the uterine arteries matters in transplant and surgical planning.

Uterine artery anatomy has been characterized in detail in human donors being screened for uterus transplantation. In a prospective study of 12 potential donors, CT angiography, digital subtraction angiography, and MR angiography gave comparable average uterine artery lumen diameters (about 2.0 to 2.1 mm), and MR angiography failed to fully evaluate 10 of 23 arteries that proved patent on digital subtraction angiography [4]. The practical point for comparative anatomy is that the uterine artery is a small-caliber vessel with a variable course, and its anastomotic connection to the ovarian artery is what makes the uterine blood supply redundant.

Within the uterine wall, the arteries branch into arcuate vessels in the myometrium and then into radial branches that supply the endometrium. The myometrial veins run alongside them, and in multiparous dogs the adventitia of those veins contains numerous layers of elastic fibers compared with only a few layers in nulliparous dogs [3]. The vascular bed of the uterus is therefore not static. It remodels with parity and with the cycle.

Uterine Shape Across Species

The gross shape of the uterus is the single most useful landmark in a comparative womb diagram. The key variable is how far the paired Müllerian ducts fused during embryogenesis. The precursor tissue of the uterus, oviducts, and upper vagina is the Müllerian duct epithelium and its adjacent mesenchyme, and the morphological differences between species are established during embryogenesis by species-specific differences in Müllerian duct fusion at the midline, growth, and differentiation [5].

Where fusion is complete, the result is a simplex uterus with a single chamber and a single cervix, as in humans. Where fusion is partial, the result is a bicornuate uterus with a body and two horns. Where fusion is minimal, the result is a bipartite uterus with two separate horns and, in some species, two cervices, as in laboratory mice [5].

Horn length relative to body length is the second variable, and it changes the practical anatomy considerably. In a polytocous species such as the rat, uterine position effects on fetal body and brain weight depend on horn size, and there is an inverse relationship between horn size and fetal weight [6]. Horn length is not a cosmetic detail. It determines how many conceptuses a horn can carry and how they are spaced.

Human: Simplex Uterus

The human uterus is a simplex organ. It has a single chamber, a single cervix, and a pear shape that is wider at the fundus and narrower at the cervix. The fallopian tubes enter at the two superolateral corners, and the round ligaments and broad ligament hold it in the pelvis. In a study comparing ovine and human pelvic anatomy for laparoscopic sacrocolpopexy, the ovine pelvic structure and vagina were similar in size to those of humans, but the dissection planes and the attachment of the posterior mesh to the uterus differed between the two models [7]. Size similarity does not mean structural identity.

Human uterine morphology is also variable within the species. Alterations in Müllerian duct development can produce uterine shape variants that correlate with increased risks of miscarriage and infertility [5]. A womb diagram of the human uterus should be read as the most common configuration, not the only one.

Dog and Cat: Bicornuate With Long Horns

The dog and cat have a bicornuate uterus with long horns and a short body. The horns extend cranially and are the site of implantation and gestation. In the dog, the horns lie in the dorsal abdomen and are suspended by the broad ligament.

The canine uterus has been studied morphometrically in detail. In Beagles, the absolute volume of the uterine wall did not change with parity, but vascular volume increased and myometrial connective tissue decreased in multiparous animals [3]. The dog is also unusual in that the non-pregnant uterus does not increase in size and volume the way it does in several other species [3]. That is a clinically useful fact: a non-pregnant canine uterus is a small, thin-walled tube, and its size on imaging or at surgery reflects that.

Canine placentation is intermediate in invasiveness between the less invasive and more invasive placental types, which places the dog in a useful comparative position for studying the maternal-fetal interface [8]. The uterus that supports that placenta is a bicornuate organ with a short body and horns that carry litters.

Horse: Bicornuate With Short Body

The mare has a bicornuate uterus with a relatively short body and two horns that curve dorsally and caudally. The equine uterus is a muscular, T-shaped organ when viewed from the dorsal aspect.

The equine endometrium is the tissue sampled by uterine biopsy and cytology, and it is the site of the maternal recognition of pregnancy. Extracellular vesicles have been isolated from equine uterine lavage fluid, and they are considered likely players in the embryo-maternal dialogue [9]. The equine uterine microbiome has also been characterized from cytobrush samples, with RNA-based 16S analysis detecting more amplicon sequence variants and taxonomic units than DNA-based analysis from the same samples [10]. Both findings point to the same anatomical reality: the equine endometrium is an active secretory surface, not a passive lining.

Endometrial structure in equids can be quantified. In mules, the stratum spongiosum was the predominant component of the endometrium, with a mean volume density of 84.7 percent, and the overall profile showed an intermediate phenotype with a clear asinine bias [11]. That is a hybrid-specific finding, but it illustrates how much of the endometrium is glandular stroma rather than surface epithelium.

Cow: Bicornuate With Prominent Intercornual Ligament

The bovine uterus is bicornuate with a short body and two horns that curve ventrally and caudally. The horns are joined at their dorsal surface by the intercornual ligament, a band of tissue that connects the two horns and is a reliable landmark during rectal palpation and at surgery.

The bovine uterus carries one calf per pregnancy in most cases, and the side of pregnancy is not random. In a retrospective study of 6,515 Holstein birth records, the incidence of right-horn pregnancy was 60.5 percent, higher than the expected 50 percent [12]. The same study found no difference in the secondary sex ratio between left and right horns, with an overall male skew of 53.1 percent [12]. The side bias is a placement phenomenon, not a sex-ratio phenomenon.

Sow: Bicornuate With Very Long Horns

The sow has a bicornuate uterus with very long horns that coil within the abdominal cavity. The horns are the site of implantation for large litters, and their length is the anatomical basis for the sow's high fecundity. The uterine body is short, and the cervix is long.

The long-horned uterus is not unique to the pig among polytocous species, but the sow's horn length is extreme. In the rat, horn size influences fetal weight, with larger horns associated with lower fetal weights [6]. The same principle of space and resource allocation applies broadly to polytocous uteri.

Species Comparison Table

Species	Uterine shape	Number of horns	Cervix
Human	Simplex, single chamber	None (single cavity)	Single
Dog	Bicornuate, long horns, short body	Two	Single
Cat	Bicornuate, long horns, short body	Two	Single
Horse	Bicornuate, short body	Two	Single
Cow	Bicornuate, prominent intercornual ligament	Two	Interlocking folds
Ewe	Bicornuate	Two	Interlocking folds
Sow	Bicornuate, very long horns	Two	Interdigitating pads

The Cervix Differs Too

The cervix is the muscular valve between the uterus and the vagina, and its internal architecture is as variable as the uterine body.

The human cervix is a single canal with a relatively smooth endocervical surface and a small external os. The dog and cat also have a single cervix, but the canine cervix is a short, thick structure that projects into the cranial vagina.

The cow and ewe have a cervix with interlocking folds. These folds are prominent ridges of tissue that interdigitate and form a tight seal. The seal is what protects the uterus from the vaginal environment between estrous periods and during pregnancy. In the ewe, the cervix is also the site of the postpartum involution process, which has been monitored radiographically. In a study of 13 suckling mule ewes, the maximum reduction in the length of the uterine body and in the diameters of the horns occurred by 28 days postpartum, except in one ewe in which involution continued for 42 days [13]. The presence of intrauterine bacteria did not change the time to complete involution in that study [13].

The sow has a cervix with interdigitating pads. These pads are arranged in rows and interlock with the spiral folds of the boar's penis during mating. The sow's cervix is long and is the site where the semen is deposited.

What the Layers Do at Different Life Stages

The three layers behave differently across the reproductive cycle, and the differences are visible on imaging. In a study of healthy women using intravoxel incoherent motion MRI, the diffusion values of the three layers of the uterine corpus increased gradually from the menstrual phase to the luteal phase, while the cervix showed its highest diffusion value in the ovulatory phase [14]. The layered structure of the uterus is not just a histological description. It has measurable functional correlates.

Ovarian steroids drive these changes. In mice, estradiol treatment increased uterine expression of mature brain-derived neurotrophic factor more than six-fold, and its low-affinity receptor NGFR declined eleven-fold from proestrus through diestrus during the estrous cycle [15]. Neurotrophin signaling is one of many steroid-responsive pathways in the uterus, and it is an example of how the endometrium and myometrium respond to the same hormonal signals in different ways.

The uterus also has a microbial dimension. Studies in humans and large animals indicate a relationship between the uterine microbiome composition and endometrial receptivity, and the analysis is technically difficult because of very low microbial biomass [10]. The equine uterine cytobrush study showed that RNA-based 16S analysis can detect less than 38 bacterial genome copies with a DNA community standard, and that RNA-based analysis has at least a ten-fold higher sensitivity than DNA-based analysis from the same samples [10]. The uterine lumen is not sterile in the absolute sense, and the organisms present are few and hard to detect.

Clinical Relevance, Limitations and Common Mistakes

The most common mistake in reading a womb diagram is assuming that the human layout is the default. It is not. The simplex human uterus is one configuration among several, and the bicornuate uterus with long horns is the norm in dogs, cats, and pigs [5].

A second mistake is treating the two horns as equivalent. In impala, the right horn showed greater epithelial height than the left in most specimens, and placentation begins only in the right horn in that species [1]. In dromedary camels, FGF7 and its receptor FGFR2IIIb were preferentially upregulated in the left uterine horn during the peri-implantation period [16]. Horn identity matters in some species, and it is not always the same horn.

A third mistake is assuming that parity leaves the uterus unchanged. In dogs, multiparity is associated with more vascular volume, less myometrial connective tissue, and arterial intimal thickening with a thinned tunica media [3]. These are real structural changes that a clinician should expect in an older, repeatedly bred animal.

A fourth mistake is underestimating the uterine blood supply's redundancy. The anastomosis between the uterine artery and the ovarian artery means that ligating one source may not devascularize the organ. In human uterus transplant screening, the uterine artery lumen averaged about 2.0 to 2.1 mm across three imaging modalities, and MR angiography missed 10 of 23 arteries that were patent on digital subtraction angiography [4]. Small vessels with variable courses are easy to miss on any single study.

Species differences also limit how far animal models can be pushed. The ovine model is useful for laparoscopic sacrocolpopexy training because the ovine pelvic structure and vagina are similar in size to those of humans, but patient positioning, trocar placement, and the attachment of the posterior mesh to the uterus differ between the two [7]. Similarity in size is not similarity in anatomy.

Individual animals vary, and a veterinarian should evaluate any specific reproductive concern with a physical examination and appropriate imaging.

Frequently Asked Questions

What are the three layers of the uterus?

The three layers are the endometrium (inner mucosa), the myometrium (middle smooth muscle), and the perimetrium (outer serous coat).

What artery supplies the uterus?

The uterine artery, a branch of the internal iliac artery, is the principal supply, and it anastomoses with the ovarian artery near the tip of the uterine horn.

Do dogs have two uteruses?

No. Dogs have one uterus with two horns, which is a bicornuate uterus, not two separate organs.

How many horns does a cow have?

A cow has two uterine horns joined by a prominent intercornual ligament.

How is the sow's uterus different from the dog's?

The sow's uterus is also bicornuate, but its horns are much longer and coil within the abdomen to accommodate large litters.

What is the difference between a simplex and a bicornuate uterus?

A simplex uterus has a single chamber and no horns, as in humans. A bicornuate uterus has a body and two horns, as in dogs, cats, horses, cows, and pigs.

Why does the cervix differ between species?

The cervix differs because its folds and pads form a seal that matches each species' reproductive anatomy and mating pattern, such as the interlocking folds of the cow and ewe and the interdigitating pads of the sow.

Does pregnancy change the uterine wall permanently?

Repeated pregnancy is associated with increased uterine vascular volume, reduced myometrial connective tissue, and arterial intimal thickening in dogs, so the wall does not return to its nulliparous structure in every respect.