Congenital Uterine Anomalies: Bicornuate, Septate, Arcuate, Unicornuate and Didelphys Uterus
A congenital uterine anomaly is a difference in the shape of the womb that is present from birth, arising while the uterus is forming in fetal life. The most commonly discussed forms are the bicornuate uterus, the septate uterus, the arcuate uterus, the unicornuate uterus and uterus didelphys. Population estimates suggest that somewhere around five in every hundred women have some form of uterine anomaly, and the proportion is higher among women investigated for repeated pregnancy loss. Many of these differences are mild, cause no symptoms at all, and are only discovered incidentally during a scan performed for another reason. Others have a real and measurable effect on implantation, on the ability of a pregnancy to grow, and on the risk of preterm birth.
The uterus develops from two tubes called the Müllerian (paramesonephric) ducts. In the first trimester of fetal life these two ducts grow towards the midline, meet, fuse into a single structure, and then the wall between them dissolves so that one cavity remains. If fusion is incomplete you get a bicornuate or didelphys uterus. If fusion happens but the dividing wall fails to dissolve you get a septate uterus. If only one duct develops properly you get a unicornuate uterus. An arcuate uterus, often described by patients as a heart shaped uterus, represents the mildest end of this spectrum and is regarded by most specialists as a normal variant rather than a disease.
This article explains how each anomaly forms, what symptoms it may or may not produce, how modern imaging distinguishes one type from another, and how each affects natural conception, IVF and pregnancy. Distinguishing a septate uterus from a bicornuate uterus matters more than any other single point in this field, because one is often treated with a short hysteroscopic operation and the other generally is not. Anything you read here is general information. The interpretation of your own scan, and any decision about surgery, belongs to the doctor who has examined you and reviewed your images directly.
How the uterus forms, and where the different anomalies come from
Understanding the embryology makes the classification straightforward rather than intimidating. Between roughly the sixth and twelfth weeks of fetal development, two Müllerian ducts (the paired tubes that become the female reproductive tract) migrate downwards and towards each other. Their lower portions fuse in the midline to create the body of the uterus, the cervix and the upper vagina, while their upper portions remain separate and become the fallopian tubes. Immediately after fusion, the tissue partition where the two ducts met begins to break down from below upwards, a process called septal resorption. When all three steps proceed normally the result is a single triangular cavity with a smooth, flat or gently rounded roof.
Each anomaly maps onto a failure at one of these steps. Failure of one duct to develop gives a unicornuate uterus, a single banana shaped cavity with one tube. Failure of the two ducts to fuse gives uterus didelphys, two entirely separate cavities each with its own cervix. Partial fusion failure gives a bicornuate uterus, with two horns sharing a lower cavity. Failure of resorption, with fusion intact, gives a septate uterus. Because the kidneys and the ureters develop alongside the Müllerian system from adjacent tissue, anomalies of the uterus are frequently accompanied by anomalies of the urinary tract, which is why a kidney ultrasound is a standard part of the assessment.
- Agenesis or hypoplasia: the uterus fails to form, or forms only rudimentarily (as in Mayer-Rokitansky-Küster-Hauser syndrome).
- Unicornuate uterus: one duct develops, the other is absent or forms a small rudimentary horn.
- Uterus didelphys: the ducts never fuse, producing two uterine bodies and usually two cervices.
- Bicornuate uterus: fusion is incomplete at the top, producing an indented outer contour and two horns.
- Septate uterus: fusion is complete but the internal partition persists, wholly or partly.
- Arcuate uterus: a shallow dip in the roof of the cavity, at the mildest end of the resorption spectrum.
Bicornuate uterus: two horns, one cervix
A bicornuate uterus is a womb whose upper portion has failed to fuse completely, so that instead of one broad chamber there are two horns joined lower down. The defining feature is external: viewed from outside, the top of the uterus (the fundus) has a visible notch or cleft rather than a smooth convex dome. This external indentation is what separates it from a septate uterus, where the outside surface is normal and only the inside is divided. The degree of separation varies widely. In a partial bicornuate uterus the horns diverge only in the upper third; in a complete bicornuate uterus the division extends down almost to the cervix.
Most women with a bicornuate uterus menstruate normally, have normal hormone levels and ovulate normally. Conception is often unaffected, because the fallopian tubes and ovaries are usually structurally intact. The clinical issue is capacity and blood supply. Each horn is narrower than a normal cavity, so as the pregnancy grows the uterus distends earlier and the muscle stretches sooner, which is associated with an increased chance of preterm labour, of the baby lying in a breech or transverse position, and of second trimester loss. Cervical weakness is somewhat more common in this group as well, and some obstetricians therefore monitor cervical length by scan through the middle of pregnancy.
Surgery for a bicornuate uterus is uncommon and is not comparable to a hysteroscopic septum operation. Uniting the two horns requires an abdominal procedure (metroplasty) that opens the muscular wall of the uterus, leaves a scar that carries a risk of rupture in later pregnancy, and requires a waiting period before conception. Most specialists reserve it for carefully selected cases after other causes of loss have been excluded. For the majority of women, management means good obstetric surveillance rather than an operation.
Septate uterus: the anomaly that most often needs treatment
A septate uterus has a normal external shape but is divided internally by a wall of tissue, the septum, hanging down from the roof of the cavity. It is the most common congenital uterine anomaly and the one most consistently linked with reproductive difficulty. The septum may be short and involve only the upper part of the cavity (partial or subseptate), or it may extend all the way to the cervix (complete). Because the outer surface is smooth and normal, a septate uterus is easy to miss on a standard two dimensional ultrasound and is frequently misreported as a bicornuate uterus if only the inside outline is examined.
The reproductive problem is not mechanical crowding so much as tissue quality. Septal tissue is relatively poor in blood vessels compared with normal uterine muscle, and the endometrium (the lining that an embryo implants into) covering it tends to be thin, less responsive to hormones and less well supplied. An embryo that implants on the septum may establish a pregnancy but then fail to develop an adequate blood supply, which is the mechanism behind the increased rate of first trimester miscarriage seen in this group. Some women with a septum also experience implantation failure without ever seeing a positive test. Our article on uterine septum and fertility goes into the treatment decision in more detail.
Hysteroscopic septum resection, sometimes called metroplasty, is a day case operation performed through the cervix with no abdominal incision. A thin telescope is passed into the cavity and the septum is divided with scissors or an energy device until the roof of the cavity is flat. Recovery is quick. It is important to be honest about the evidence: while the operation is technically straightforward and widely performed, high quality randomised data on whether it improves live birth rates are limited, and professional bodies including ESHRE have called for caution rather than routine surgery in every case. The decision is individual, and depends on septum length, obstetric history and what else has been found.
Arcuate uterus and the heart shaped uterus question
An arcuate uterus has a shallow, smooth indentation in the roof of the cavity, with a normal external contour. It sits at the mildest point of the resorption spectrum and is best understood as a normal anatomical variant. When a scan report mentions a heart shaped uterus, an arcuate uterus is usually what is meant, although the same phrase is sometimes used loosely for a bicornuate uterus, which is a different and more significant finding. If your report uses that phrase, it is worth asking specifically which of the two was seen, and whether the outer surface of the uterus was assessed.
Formal classification systems set numerical thresholds to distinguish an arcuate from a small septum, based on the depth of the indentation and the angle it makes. Different systems draw the line in different places, which is why the same uterus can be labelled arcuate by one radiologist and subseptate by another. What matters clinically is that a genuinely shallow, wide, rounded indentation is not associated with a meaningful reduction in fertility and does not require surgery. Deep, narrow, sharply angled indentations behave differently. This is a case where the shape of the dip matters more than the label attached to it.
- An arcuate uterus does not usually cause painful or heavy periods.
- It does not usually prevent conception or require any change to an IVF plan.
- It does not usually require hysteroscopic surgery.
- If you have had repeated losses and an arcuate uterus is the only finding, the anomaly is unlikely to be the whole explanation and a wider assessment is appropriate.
Unicornuate uterus: one horn, and what to check for
A unicornuate uterus develops when one Müllerian duct fails to form properly. The result is a smaller, elongated, laterally deviated cavity with a single fallopian tube. In a substantial proportion of cases there is also a rudimentary horn on the other side, a small underdeveloped structure that may or may not contain a functioning endometrial cavity, and that may or may not communicate with the main uterus. Identifying whether a rudimentary horn is present, and whether it has an active lining, is one of the more important tasks in the assessment because it changes both symptoms and safety.
A non communicating rudimentary horn with functioning endometrium traps menstrual blood each cycle, causing progressively worsening cyclical pelvic pain that typically begins soon after periods start and is often mistaken for endometriosis. Rarely, sperm can migrate across the pelvis and a pregnancy can implant inside such a horn. Because the horn has thin muscular walls and no route out, this situation carries a genuine risk of rupture in the second trimester and is treated as an emergency. For this reason, a symptomatic or cavity containing rudimentary horn is often removed laparoscopically before a woman tries to conceive.
The main uterus in a unicornuate configuration is smaller than average, which is associated with a higher chance of preterm birth, of the baby growing more slowly than expected, and of abnormal fetal position. Because a unicornuate uterus is often accompanied by an absent or malpositioned kidney on the same side, imaging of the urinary tract is routine. Fertility itself may be preserved, since one normal tube and ovary can be enough, but tubal patency on the functioning side becomes critical, and a blocked single tube changes the treatment plan substantially.
Uterus didelphys: two separate cavities
Uterus didelphys is complete non fusion: two distinct uterine bodies, each with its own cervix, and in many cases a longitudinal vaginal septum dividing the vagina into two channels. Women sometimes discover this in adolescence when a tampon fails to stop bleeding, because it has been placed in one channel while the other continues to bleed. Others discover it when a routine smear test proves difficult, or during a first pregnancy scan. Intercourse is usually comfortable, though a thick vaginal septum can cause pain and is sometimes divided surgically.
A specific and important variant is obstructed hemivagina with ipsilateral renal agenesis, in which one of the two vaginal channels is blocked and the kidney on that side is absent. Menstrual blood accumulates behind the obstruction, producing severe cyclical pain and a palpable pelvic mass in a young girl who nonetheless appears to be having normal periods from the other side. This is a treatable condition once recognised, but it is frequently diagnosed late. Any adolescent with severe progressive period pain and a known single kidney warrants dedicated imaging of the reproductive tract.
In reproductive terms, each cavity is smaller than a normal uterus and pregnancy usually establishes in one of them. Conception rates are often reasonable, but preterm birth and malpresentation are more frequent, and caesarean delivery is more common. Surgery to join the two cavities is rarely performed and is not generally recommended, since the two hemi uteri are separately formed structures with their own blood supply rather than a single uterus with a partition in it.
How congenital uterine anomalies are diagnosed
Accurate diagnosis depends on seeing both the inside of the cavity and the outside contour of the uterus in the same examination. This is the single most common failure in practice. A standard two dimensional pelvic ultrasound, a hysterosalpingogram (an X-ray with dye passed through the cervix) and diagnostic hysteroscopy all show the internal cavity well, but none of them shows the external fundal surface. A septate uterus and a bicornuate uterus can look identical on all three. That is why a report describing two horns based on dye X-ray alone should be regarded as provisional until better imaging is done.
Three dimensional transvaginal ultrasound has become the first line investigation. It reconstructs a coronal view of the uterus, showing the cavity outline and the outer contour together in a single image, without radiation or contrast, and is best performed in the second half of the cycle when the lining is thick and the cavity outline is clear. Saline infusion sonography, where a small amount of sterile fluid is instilled to open the cavity, adds further clarity. Pelvic MRI is reserved for complex cases, for suspected rudimentary horns, and for anomalies involving the cervix or vagina. Kidney imaging accompanies all of these when a fusion anomaly is confirmed.
- 3D transvaginal ultrasound: shows cavity and external contour together; first line in most units.
- Saline infusion sonohysterography: distends the cavity for a clearer internal outline.
- Hysterosalpingography (HSG): shows cavity shape and tubal patency, but not the outer contour.
- Pelvic MRI: best for complex anomalies, rudimentary horns and vaginal or cervical involvement.
- Hysteroscopy with laparoscopy: the historical reference standard, now used mainly when surgery is planned anyway.
- Renal ultrasound: routine, because urinary tract anomalies frequently accompany Müllerian anomalies.
Symptoms: what women actually notice
Most congenital uterine anomalies produce no symptoms whatsoever. A woman with a septate or arcuate uterus typically has entirely normal periods and no pelvic pain, and there is nothing on physical examination to suggest anything unusual. This is why so many are found incidentally, during an early pregnancy scan, a fertility workup, or imaging performed for an unrelated problem. The absence of symptoms says nothing about whether an anomaly is present, and equally, the presence of an anomaly does not mean that every gynaecological symptom you have is caused by it.
Symptoms, when they occur, usually indicate obstruction. Menstrual blood that cannot escape causes pain that builds through the days of bleeding and worsens month by month over years. That pattern, in an adolescent, should prompt imaging rather than reassurance. Difficulty with tampons, bleeding that continues despite a tampon in place, or pain at a particular point during intercourse can point to a vaginal septum. Recurrent early pregnancy loss, particularly loss occurring at a similar gestation each time, and unexplained second trimester loss or very preterm delivery are the reproductive presentations that most often lead to diagnosis.
- Severe, progressively worsening period pain starting in adolescence.
- Bleeding that continues around a correctly placed tampon.
- Pain or a sensation of obstruction with intercourse or with a speculum examination.
- Two or more early pregnancy losses, or a loss in the second trimester.
- Preterm labour, breech or transverse presentation in a previous pregnancy.
- A known single kidney or other urinary tract anomaly.
How each anomaly affects fertility and pregnancy
It helps to separate three distinct questions: can an embryo implant, can a pregnancy continue through the first trimester, and can the uterus carry a pregnancy to term. Most anomalies affect the second and third far more than the first. Ovulation, egg quality and hormone production are governed by the ovaries, which develop from different tissue and are usually entirely normal in women with Müllerian anomalies. So the common assumption that a differently shaped uterus means poor fertility overall is not accurate. Many women with these anomalies conceive without difficulty.
Where the anomaly does interfere, the mechanisms differ by type. A septum affects implantation and early placental development through poor vascularity of the septal surface. Bicornuate, unicornuate and didelphys uteri affect the later stages by limiting the volume and distensibility of the cavity, which is why preterm birth and fetal malposition dominate their risk profile. An arcuate uterus, in most analyses, does not have a clinically meaningful effect. Where recurrent loss occurs, other contributors should still be looked for; anomalies coexist with thyroid disease, clotting problems, chromosomal factors and chronic endometritis, and finding one does not exclude the others.
Pregnancy in a structurally different uterus is usually managed as higher risk obstetric care rather than as a contraindication to pregnancy. That typically means confirming the location of the pregnancy early, serial growth scans in the second half of pregnancy, cervical length monitoring in some cases, planning for the possibility of preterm delivery, and discussing mode of delivery in advance because malpresentation and caesarean birth are more common. These are practical adjustments, not reasons for alarm.
Uterine anomalies and IVF
An anomaly does not by itself make IVF necessary. IVF becomes relevant when it is indicated for other reasons: blocked tubes, low ovarian reserve, a male factor, or persistent unexplained infertility. In a unicornuate uterus with a blocked single tube, or in a didelphys uterus where access to the pregnancy side is difficult, IVF may be the practical route. Where IVF is used, the ovarian stimulation and egg collection phases proceed exactly as they would in any other patient, because the ovaries are unaffected by uterine shape. You can read about the sequence of steps in our overview of the IVF process.
Where the anomaly does change practice is at transfer. A distorted or divided cavity makes catheter placement less predictable, so ultrasound guidance is standard and the operator plans in advance which cavity, and which part of it, is being targeted. In a septate uterus, if resection has been agreed, it is generally completed and the cavity confirmed healed before transfer rather than afterwards. Single embryo transfer is strongly preferred in a small or divided cavity, because a twin pregnancy in a uterus with reduced capacity substantially increases the risk of very preterm birth. Frozen transfer allows the timing and the lining to be optimised separately from stimulation.
If transfers have failed despite good quality embryos, the cavity deserves a careful second look. Repeated implantation failure is a situation where a previously undiagnosed septum, adhesions or an inflamed lining are sometimes uncovered. The relevant question is not simply whether an anomaly exists, but whether the endometrial surface where the embryo is being placed is healthy and adequately supplied. Our article on recurrent miscarriage and implantation failure covers the broader workup.
Which anomalies are treated, and which are not
Surgical decisions in this field follow a clear logic: operate when the abnormal tissue itself is causing the problem and removing it restores normal anatomy with acceptable risk. That description fits a uterine septum, which can be divided hysteroscopically without cutting the uterine wall, and fits an obstructing vaginal septum or a symptomatic rudimentary horn. It does not fit a bicornuate or didelphys uterus, where the anatomy is not a partition to be removed but two separately formed structures, and where unification requires major surgery with a scarred uterus as the result.
It equally does not fit an arcuate uterus, where there is no convincing evidence that intervention improves outcomes and where operating introduces the risk of intrauterine adhesions for no clear benefit. Whenever hysteroscopic surgery is performed inside the cavity, adhesion formation is a recognised complication, and a follow up hysteroscopy or scan to confirm that the cavity has healed cleanly is a sensible part of the plan. This is one reason not to operate on findings of doubtful significance.
- Usually considered for surgery: significant uterine septum with a history of loss or implantation failure; obstructing vaginal septum; symptomatic or cavity containing rudimentary horn.
- Usually not operated: arcuate uterus; asymptomatic bicornuate uterus; uterus didelphys without obstruction.
- Individually judged: a short septum found incidentally with no reproductive history, where observation is often reasonable.
When to seek assessment, and what to ask
Seek a gynaecological assessment if you have had two or more pregnancy losses, any loss after the first trimester, a previous very preterm delivery, or period pain that has been progressively worsening since adolescence and is not controlled by simple measures. Seek urgent care in pregnancy for severe one sided abdominal pain, particularly if a rudimentary horn has previously been mentioned on any scan. If you are already having fertility investigations, ask directly whether your uterine cavity and external contour have both been imaged, and by what method.
Useful questions in the consultation include: which specific anomaly do my images show, and how confident is that distinction; was the outer surface of my uterus seen or only the cavity; have my kidneys been checked; does this finding explain my history, or are we still looking for other causes; and if surgery is being suggested, what is the expected benefit in my particular situation as opposed to in general. A good discussion should leave you clear about which parts of the plan are well supported and which are judgement calls. Baseline fertility testing is described in our guide to fertility testing.
Finally, keep proportion. Many women with a congenital uterine anomaly have straightforward pregnancies with no intervention beyond ordinary antenatal care. The purpose of diagnosis is not to create anxiety but to allow the small number of situations that genuinely need action, an obstructed horn, a significant septum, a cervix that needs watching, to be identified in advance rather than in an emergency. Discuss your own imaging and history with your treating doctor, who can weigh the specific findings against your circumstances.
Related Reading
- Uterine Septum and Fertility: What to Know
- Recurrent Miscarriage and Implantation Failure: Causes and What Can Help
- Fertility Testing: How to Know If You Are Fertile (Female and Male)
- Endometrial Lining and Implantation: What Matters
- Anteverted Uterus: What It Means, and Whether It Affects Fertility
- Enlarged Uterus: Causes, Symptoms and Fertility Implications
Sources and references
This article is supported by the following independent, authoritative sources: