What Is a Uterine Horn? Human & Animal Anatomy

A uterine horn is one of the two upper extensions of the uterus where it meets the fallopian tubes. In most mammals, these horns are large, prominent structures where embryos implant and develop. In humans, the uterus has fused into a single pear-shaped organ during evolution, so the “horns” are just the small upper corners, called cornua, where the cavity narrows and opens into each fallopian tube. The term shows up in two very different contexts: veterinary and comparative anatomy, where horns are a defining feature of uterine shape, and human medicine, where the word usually surfaces in discussions of congenital anomalies that leave a person with an incompletely fused or one-sided uterus.

The Human Uterine Horn in Normal Anatomy

In a typical human uterus, the internal cavity is roughly triangular, measuring about 3.5 cm in length, and at each upper corner the cavity narrows into a small projection called the cornu, or horn.¹ Each cornu connects to a fallopian tube, creating the passageway through which an egg travels from the ovary into the uterine cavity. For most people, these cornua are barely noticeable on imaging and have no clinical significance at all. The human uterus is classified as “simplex,” meaning the two embryonic ducts that formed it fused completely during fetal development, leaving a single cavity with no distinct right or left horn.

Why Most Mammals Have Prominent Uterine Horns

Humans are the exception here. The vast majority of mammals retain two separate uterine horns to varying degrees, and the shape of those horns tracks closely with how many offspring a species carries at once. Comparative anatomy groups mammalian uteri into several broad types based on how much fusion occurred between the two embryonic ducts that give rise to the reproductive tract.

• Simplex: A fully fused, single-cavity uterus. Found in humans and some other primates.
• Bicornuate: Two distinct horns that merge into a shared body and cervix. This is the arrangement in cows, sheep, pigs, dogs, and cats.
• Bipartite: Two horns separated by a septum that extends almost to the cervix. Found in horses.
• Duplex: Two completely separate uteri, each with its own cervix. Rats, rabbits, and guinea pigs have this arrangement.

These categories come from comparative anatomy teaching and are widely used in veterinary science.² Not every mammal fits neatly into one box, though. Marsupials like the sugar glider have a completely different setup: paired uteri that lack horns altogether, plus a vaginal complex with two lateral vaginae and a separate median vagina that functions as the birth canal.³ Reptiles that bear live young also have paired reproductive tracts, and researchers have found that the uterine lining in pregnant skinks actively regulates which nutrients pass through to the embryo, with specialized proteins tightening the junctions between cells late in pregnancy to control ion flow.⁴

The reason horn shape matters biologically is simple: species that produce large litters need physical space along the length of the horns for each embryo to implant and grow. A sow’s uterine horns can be over a meter long, providing enough room for a dozen or more piglets. A cow’s two shorter horns typically carry just one calf. The horn is not just a passive container. It plays an active role in spacing embryos out, contracting in coordinated waves, and nourishing each implantation site independently.

How Embryos Use the Horn

In species like mice and rats, where each horn hosts multiple embryos, the uterus actively positions those embryos along the available length. Research in mice has revealed three distinct stages of pre-implantation movement. First, embryo clusters enter the horn. Then the uterus moves them in one direction through adrenergic smooth-muscle contractions, essentially squeezing them along like a conveyor belt. Finally, the embryos scatter and space themselves out through a chemical signaling pathway that depends on communication between the embryos and the uterine wall.⁵ The result is that implantation sites end up roughly evenly spaced rather than clumped at one end. Researchers have proposed that the uterus effectively “counts” the embryos as a cluster before scattering them to match the available horn length.

Horn size also influences fetal growth. In rats carrying large litters of more than eight pups, fetal body weight follows a U-shaped curve depending on position within the horn: pups near the ovary end and the cervix end tend to be heavier, while those in the middle are lighter. In smaller litters of fewer than four, the relationship is more straightforward and linear.⁶ This suggests that blood supply and nutrient delivery along the horn are not uniform, and that crowding in the middle section limits growth when many embryos compete for space.

Uterine Horn Anomalies in Humans

When human uterine horns come up in a medical context, it almost always involves a Müllerian duct anomaly, a congenital variation where the two ducts that form the uterus during fetal development did not fuse completely or did not form at all. The Müllerian ducts normally fuse into a single uterine body between about the eighth and twelfth week of fetal life. Several genes and signaling pathways govern this process, and disruptions at different stages produce different shapes.⁷

The American Society of Reproductive Medicine now classifies these anomalies into nine descriptive categories, and there is also a separate European classification system.⁸ The ones most relevant to uterine horns include:

• Unicornuate uterus: Only one Müllerian duct developed fully, producing a single banana-shaped horn on one side. The other side may have a small, underdeveloped “rudimentary” horn, or nothing at all.⁹
• Bicornuate uterus: The two ducts fused only partially, leaving two distinct upper horns that share a cervix. The degree of separation varies from a small notch at the top to two nearly complete horns.
• Didelphys uterus: Complete failure of fusion, resulting in two separate uteri, each with its own cervix and sometimes its own vagina, essentially a duplex arrangement similar to what rabbits and rodents have naturally.

Diagnosis typically relies on MRI, which is considered the best imaging method for sorting out the anatomy, though three-dimensional transvaginal ultrasound has shown substantial agreement with MRI across most categories of anomaly.¹⁰ Many people with these anomalies never know they have one until they have imaging done for another reason or experience complications during pregnancy.

Pregnancy in an Abnormal Horn

The clinical significance of uterine horn anomalies centers heavily on pregnancy. A bicornuate uterus can carry a pregnancy to term, but the risk of complications is elevated: fetal malpresentation, preterm delivery, restricted fetal growth, recurrent pregnancy loss, and cervical incompetence all occur more frequently than in a normal uterus.¹¹ One review of outcomes estimated that pregnancies in a bicornuate uterus are associated with roughly a 21% miscarriage rate, a 24% prematurity rate, and an 84% rate of cesarean delivery.¹²

The picture is more challenging when pregnancy occurs in a unicornuate uterus. A retrospective study comparing pregnancies in a hemi-uterus to pregnancies in a normal uterus found substantially higher rates of prior spontaneous abortion (about 24% versus 11%), and the risks of preterm birth, preterm premature rupture of membranes, malpresentation, and cesarean delivery were all markedly elevated.¹³ The smaller cavity simply cannot expand the way a full-sized uterus can, leading to crowding and premature labor.

The most dangerous scenario involves pregnancy in a rudimentary horn, the small, underdeveloped remnant that sometimes exists alongside a unicornuate uterus. If the horn has a functioning endometrial lining but does not communicate with the main uterine cavity, an embryo that implants there is in trouble. The muscular wall of a rudimentary horn is thin and cannot stretch enough to accommodate a growing pregnancy. Rupture, usually in the second trimester, is a life-threatening emergency.¹⁴ Because rudimentary horn pregnancies are rare, they are often diagnosed late. The standard treatment is surgical removal of the horn.¹⁵

Surgical Options for Problematic Horns

When a rudimentary horn causes recurring pain, typically because menstrual blood is produced inside it but has no way to drain, laparoscopic removal is the standard approach. In a series of patients who underwent laparoscopic excision of a non-communicating cavitated horn, all had complete resolution of their pain afterward. Among those who later attempted pregnancy, outcomes were mixed: two pregnancies ended in first-trimester loss, while two others reached 34 and 36 weeks before premature delivery by cesarean, with no serious complications.¹⁶ The technical difficulty of the surgery depends on how the horn is attached: a thin fibrous band connecting it to the main uterus is easier to manage than thick muscular tissue, which may require suturing the uterine wall closed to prevent rupture risk in a future pregnancy.¹⁷

A more ambitious approach is metroplasty, where a surgeon unifies the rudimentary horn with the main uterine cavity to create a single larger space. A small case series reported success in five patients, with all experiencing relief from pain and resumption of regular menstruation. One patient later became pregnant through IVF and delivered twins by cesarean at 33 weeks.¹⁸ These surgeries remain uncommon and technically demanding, and outcomes data are limited to small case series rather than large trials.

Kidney Anomalies That Travel With Horn Anomalies

One thing that catches many people off guard is that uterine anomalies often come with kidney problems. The embryonic structures that form the kidneys and ureters develop alongside the Müllerian ducts, so a disruption in one system frequently means a disruption in the other. In a study of 376 women with Müllerian duct anomalies, about 17% also had kidney or urinary tract abnormalities. The rate was highest in women with a unicornuate or didelphys uterus, where roughly 29% had an associated renal anomaly.¹⁹

A separate review found that nearly a third of patients with Müllerian duct anomalies were missing a kidney entirely, and among those, the majority also had additional complications like ectopic ureters draining into the vagina or cervix.²⁰ This is why guidelines generally recommend imaging the kidneys whenever a uterine anomaly is found, even if the person has no urinary symptoms.

Uterine Horns in Veterinary Medicine

In veterinary practice, uterine horns are part of everyday anatomy rather than a rare anomaly. When a dog or cat is spayed, the procedure, called ovariohysterectomy, involves removing both ovaries and the entire uterus including both horns. The surgery is typically performed through an incision along the midline of the abdomen.²¹ Each horn has its own blood supply that must be carefully tied off or sealed, and vessel-sealing devices used during the procedure sometimes need to be applied multiple times at each site to ensure the seal holds.²²

Uterine horns are also where pyometra, a potentially fatal uterine infection, develops in unspayed dogs and cats. This progesterone-dependent condition causes the uterine horns to fill with pus, sometimes expanding them to several times their normal diameter.²³ Research has clarified that pyometra can develop either as a progression from cystic endometrial hyperplasia, where the uterine lining develops abnormal cysts under the influence of progesterone, or as a standalone infection without prior cystic changes.²⁴ Either way, the horns bear the brunt of the disease and are the structures that must be removed to cure it.

Horn Length and Litter Size in Pigs

In pig production, the physical capacity of the uterine horns has long been recognized as a limiting factor on litter size. A sow may ovulate twenty or more eggs, but if the combined length and blood supply of her two horns cannot support that many embryos, some will be lost early in pregnancy. Researchers have studied this using surgical models where one horn or one ovary is removed before breeding. In a recent study, limiting uterine space by ligating one oviduct reduced overall litter size but improved the survival and weight uniformity of the embryos that remained, suggesting that each embryo had more resources.²⁵ However, removing one horn entirely increased early embryonic loss, probably because the remaining horn was suddenly overcrowded.

Attempts to increase litter size by selecting for longer uterine horns have had limited success. One long-running genetic selection experiment found no significant changes in uterine dimensions despite selecting for larger litters.²⁶ The overall picture seems to be that uterine capacity involves more than just horn length: blood supply, placental efficiency, and endometrial health all play a role, and the genetics are complex. Researchers have described uterine capacity as reflecting a combination of the uterine environment and the genetic makeup of the embryos themselves.²⁷

How Infection Alters Horn Function

Uterine horns are not just passive containers for pregnancy; they are muscular organs that contract rhythmically to move fluid, debris, and sperm. Infections can disrupt this contractile behavior in ways that matter for fertility. In a mouse model, chlamydial infection increased the force of contractions in the uterine horn by 53 to 83% in response to prostaglandin, a hormone-like compound that normally triggers uterine contractions. The response was specific: contractions driven by other signals like histamine were unaffected.²⁸ This kind of altered contractility could interfere with embryo transport and implantation, offering one mechanism by which chlamydial infections impair fertility even after the active infection is treated.

The uterine horn and cervix also responded differently to the same infection, with the cervix showing distinct contractile changes. This is a good reminder that even within a single reproductive tract, different regions have their own smooth muscle properties and respond independently to disease. Researchers developing treatments for infection-related infertility need to account for these regional differences rather than treating the uterus as a uniform organ.