Congenital Uterine Anomalies

• A septate uterus is the most common anomaly and carries the worst reproductive outcomes — and it is the only one correctable through a transcervical, hysteroscopic operation
• A bicornuate uterus looks nearly identical on a hysterosalpingogram but cannot be fixed that way; correcting it would require an abdominal metroplasty that is very rarely justified
• A unicornuate uterus with a non-communicating rudimentary horn is the one that can kill a patient, through pregnancy in the horn and second-trimester rupture
• An arcuate uterus is a normal variant and needs nothing at all

Three sequential steps, three families of failure

• Step 1 — Development. Both ducts elongate caudally. Complete bilateral failure gives uterine agenesis (MRKH); unilateral failure gives a unicornuate uterus, with or without a rudimentary horn on the failed side
• Step 2 — Fusion. The caudal segments of the two ducts meet and fuse in the midline. Complete failure gives uterus didelphys — two separate uteri, two cervices, usually a longitudinal vaginal septum. Partial failure gives a bicornuate uterus, where the lower segment fused but the fundus did not, producing an externally indented fundus
• Step 3 — Resorption. The septum between the now-fused ducts dissolves from caudal to cephalad, creating one cavity. Failure gives a septate uterus: the outside is normal because fusion succeeded, but a fibromuscular septum divides the cavity. Incomplete resorption at the very top leaves an arcuate uterus

Why septate is the one that damages pregnancies

• The septum is relatively avascular and fibromuscular, with poor vascularity and abnormal endometrial differentiation over its surface
• An embryo implanting on the septum meets inadequate blood supply and defective decidualization, producing early pregnancy loss
• The reduced and distorted cavity volume also drives preterm birth, malpresentation and second-trimester loss in pregnancies that continue
• This mechanism is specific to the septum. A bicornuate or didelphic uterus has normal myometrium and endometrium throughout, and its problems are volumetric — space and cervical competence — rather than implantation

Why unicornuate is the most obstetrically dangerous

• A single small cavity gives the highest aggregate rate of preterm birth, malpresentation and fetal growth restriction of any anomaly
• The associated rudimentary horn is the specific hazard. If it contains functional endometrium and does not communicate with the main cavity, sperm can still reach it by transperitoneal migration, producing a pregnancy in a thin-walled horn that ruptures — typically in the second trimester, with catastrophic hemorrhage
• Because one duct failed, the adjacent mesonephric (renal) system frequently failed with it, which is why renal anomalies cluster with unilateral and obstructive defects

The DES exception

• In utero diethylstilbestrol exposure produced a T-shaped uterine cavity, cervical hood and hypoplastic uterus, plus vaginal clear cell adenocarcinoma. US use ended in 1971, so exposed women are now past reproductive age and this is largely historical — but it remains the one acquired-in-utero uterine anomaly and is still examined

Classification

• ASRM 2021 Müllerian Anomaly Classification (MAC) is the current US standard, replacing the 1988 AFS system. It categorizes: Müllerian agenesis, cervical agenesis, unicornuate, uterus didelphys, bicornuate, septate, longitudinal/transverse vaginal septum, and complex/ unclassified
• ESHRE/ESGE (2013) is the parallel European system, using U0-U6 for the uterus with separate cervical and vaginal subclasses. It defines septate more liberally than ASRM, so the two systems disagree on borderline cases — a real source of conflicting reports in the literature
• The ASRM 2021 morphometric criteria for a septate uterus: an internal indentation depth exceeding 1 cm with an apical angle under 90 degrees, and an external fundal contour that is flat or indented by less than 1 cm

Relative frequency among anomalies

• Septate — roughly 35-55%, the most common
• Bicornuate — roughly 10-25%
• Arcuate — commonly reported at 5-20%, though many series now count it as a normal variant rather than an anomaly, which is why the published percentages swing so widely
• Didelphys — roughly 5-10%
• Unicornuate — roughly 5-10%, of which the majority have a rudimentary horn and a substantial minority of those have functional endometrium

Associated conditions

• Renal anomalies accompany a substantial proportion, strongly weighted toward unicornuate, didelphys and obstructive defects: unilateral renal agenesis most often, then ectopic, pelvic or horseshoe kidney. Symmetric anomalies (septate, arcuate) carry little renal association
• Skeletal anomalies, particularly vertebral, in the same subgroup
• Longitudinal vaginal septum in most patients with didelphys and some with bicornuate
• Endometriosis, when any component is obstructed and menstrual blood refluxes

Risk factors

• The great majority are sporadic, with no identified exposure. Familial clustering exists and points to polygenic inheritance with incomplete penetrance
• No modifiable maternal exposure has been established apart from diethylstilbestrol
• Being investigated for infertility or recurrent pregnancy loss is itself the strongest practical predictor, because that is the population in which anomalies are found

Usually nothing at all

• Most women with a uterine anomaly are asymptomatic, menstruate normally, conceive normally and deliver normally. Incidental discovery on imaging performed for another reason is the most common "presentation"
• Anomalies do not cause primary amenorrhea — the endometrium and outflow tract are intact. Primary amenorrhea points to agenesis or obstruction instead

When they do present

• Recurrent pregnancy loss, most characteristically first- and early second-trimester, and most characteristically with a septate uterus
• Infertility, though the causal link is weaker than for pregnancy loss and anomalies are often a coincidental finding in an infertility work-up
• Preterm birth, malpresentation (breech and transverse), fetal growth restriction, cervical insufficiency, and retained placenta — the obstetric signature, most pronounced in unicornuate and bicornuate uteri
• Dysmenorrhea and dyspareunia, particularly with a longitudinal vaginal septum (which may also cause the report of bleeding through a tampon that is "already in place" — a second, unoccluded hemivagina)
• Acute abdomen in the second trimester from rupture of a rudimentary horn pregnancy — hemodynamic collapse in a pregnant patient with known or unknown unicornuate anatomy. This is a surgical emergency and carries high mortality if not recognized
• Cyclic pain with otherwise normal menses when one component is obstructed — see the obstructive anomalies article; the unobstructed side keeps menstruating, which is why the history sounds reassuring

Examination

• Speculum examination may reveal two cervices (didelphys) or a longitudinal vaginal septum, both of which are easily missed if not specifically looked for
• The abdominal and bimanual examination is usually unremarkable; the uterus is not reliably palpable as abnormal
• A normal examination excludes nothing — imaging makes these diagnoses

The one question the imaging must answer

• Hysterosalpingography (HSG) outlines the cavity only. It shows two horns and cannot distinguish septate from bicornuate. It remains useful for tubal patency and often raises the initial suspicion, but an HSG report of "bicornuate uterus" should be treated as an unconfirmed hypothesis
• 3D transvaginal ultrasound is first-line and is now the reference standard for most patients. It renders the coronal plane, so it displays the external fundal contour and the internal indentation together, allowing the ASRM morphometric criteria to be applied directly. It is accurate, inexpensive and needs no contrast. Best performed in the luteal phase, when a thick endometrium outlines the cavity
• Saline infusion sonohysterography distends the cavity and improves definition, and combined with 3D imaging is highly accurate
• Pelvic MRI is reserved for complex, obstructive or equivocal anatomy, for rudimentary horns where functional endometrium must be identified, and where the renal tract is being assessed at the same time
• Combined laparoscopy and hysteroscopy was the historical gold standard — laparoscopy for the outside, hysteroscopy for the inside. It is now largely superseded by 3D ultrasound and MRI and is reserved for cases proceeding to surgery anyway

Always image the kidneys

• Renal ultrasound in every patient with a unilateral, didelphic or obstructive anomaly, and reasonably in all newly diagnosed anomalies. Unilateral renal agenesis is common enough to change lifelong care and, conversely, an incidentally found solitary kidney should prompt uterine imaging

Reading the findings

• Septate — external contour normal or indented less than 1 cm; cavity divided by a septum with indentation over 1 cm and an apical angle under 90 degrees; one cervix
• Bicornuate — external contour indented 1 cm or more; two divergent horns; one cervix, occasionally two
• Didelphys — two entirely separate uterine bodies, two non-communicating cavities, and two cervices; usually a longitudinal vaginal septum
• Unicornuate — a single elongated, laterally deviated horn with one small cavity, with or without a contralateral rudimentary horn; one cervix
• Arcuate — normal external contour with a smooth, shallow fundal indentation of the cavity; one cervix

• In a rudimentary horn, the two features that decide management are whether it contains functional endometrium and whether it communicates with the main cavity. MRI answers both

Differential

• Intrauterine adhesions (Asherman syndrome) — acquired, follows instrumentation or infection, irregular cavity filling defects, hypomenorrhea
• Submucosal leiomyoma or endometrial polyp — a filling defect, not a septum; the fundal contour is normal and the lesion is focal
• Müllerian agenesis — no cavity at all, with primary amenorrhea
• Obstructive anomalies — cyclic pain and hematometra dominate the picture

The default is observation

• Arcuate uterus: no treatment. It is a normal variant with essentially normal reproductive outcomes, and operating on it is a recognized error
• Bicornuate, didelphys and unicornuate uteri in asymptomatic women with no adverse reproductive history: no surgery. There is no evidence that prophylactic correction improves outcomes, and abdominal metroplasty introduces adhesions, a uterine scar, and a requirement for cesarean delivery

Septate uterus — the contested one, stated honestly

• Hysteroscopic septum resection (metroplasty) is technically straightforward, transcervical, and low-morbidity, and a large body of observational data reports substantially reduced miscarriage rates afterward
• However, the TRUST randomized controlled trial compared septum resection with expectant management and did not demonstrate improved live birth rates. The randomized evidence therefore does not confirm what the observational evidence suggested
• The current position is individualized shared decision-making: resection is reasonable to offer in recurrent pregnancy loss or before assisted reproduction, with the patient explicitly informed that randomized data have not shown a live-birth benefit. It should not be presented as established therapy, nor dismissed
• Complications of resection: uterine perforation, fluid overload from the distension medium, intrauterine adhesions, and thinning of the fundal wall

Unicornuate uterus with a rudimentary horn — the one with a firm indication

• Excise a rudimentary horn that contains functional endometrium, whether or not it communicates. This prevents horn pregnancy and rupture, and relieves cyclic pain and progressive endometriosis. Laparoscopic excision is standard
• A non-functional horn requires no intervention
• The main unicornuate cavity itself cannot be enlarged; management is obstetric surveillance

Didelphys and longitudinal vaginal septum

• The uterus itself is left alone; didelphic uteri often have acceptable obstetric outcomes
• Resect a longitudinal vaginal septum when it causes dyspareunia, obstructs a tampon, or would obstruct delivery
• If one hemivagina is obstructed, that is OHVIRA and requires urgent septal resection — covered in the obstructive anomalies article

Pregnancy management

• Serial cervical length surveillance in the second trimester, given elevated rates of cervical insufficiency and preterm birth
• Cerclage only for standard indications — history-indicated or ultrasound-indicated — not routinely for the anomaly itself
• Anticipate and plan for malpresentation; a higher cesarean rate is expected, and external cephalic version is less likely to succeed in a distorted cavity
• Vaginal delivery is appropriate after hysteroscopic septum resection; after abdominal metroplasty, cesarean delivery is indicated because of the myometrial scar
• Anticipate retained placenta and postpartum hemorrhage, which occur more often in anomalous uteri

What not to do

• Do not accept an HSG diagnosis of "bicornuate" and proceed to surgery — confirm the external contour first
• Do not resect an arcuate uterus
• Do not perform abdominal metroplasty for a bicornuate uterus in a woman without a documented adverse reproductive history

• Recurrent first- and second-trimester pregnancy loss, most pronounced with a septate uterus
• Preterm birth and cervical insufficiency, highest in unicornuate and bicornuate uteri
• Malpresentation — breech and transverse lie — with the resulting higher cesarean rate, and increased risk of uterine rupture in labor where a prior metroplasty scar exists
• Fetal growth restriction from reduced cavity volume and altered uterine perfusion
• Rudimentary horn pregnancy with rupture — typically second-trimester, presenting as hemoperitoneum and shock, historically with high maternal mortality and still a genuine emergency
• Retained placenta, abnormal placentation and postpartum hemorrhage
• Endometriosis and chronic pelvic pain where an obstructed component refluxes menstrual blood
• Surgical complications: uterine perforation, distension-medium fluid overload, and intrauterine adhesions after hysteroscopic metroplasty; adhesive disease, a mandatory cesarean, and impaired fertility after abdominal metroplasty
• Renal sequelae in patients with a solitary or ectopic kidney — hypertension, hyperfiltration injury, and reduced reserve
• Diagnostic harm: operating hysteroscopically on a misclassified bicornuate uterus risks perforating the fundus between two horns; leaving a functional rudimentary horn in place risks a rupture that was entirely preventable

• The septate uterus is both the most common anomaly and the one with the worst reproductive outcomes — and the only one repaired through the cervix rather than the abdomen
• The external fundal contour is the whole discriminator. Septate = normal outside, divided inside. Bicornuate = indented outside (1 cm or more). An HSG shows only the inside and therefore cannot tell them apart
• 3D transvaginal ultrasound is first-line because it renders the coronal plane and shows the outside and inside together; MRI for complex, obstructive or rudimentary-horn anatomy
• A rudimentary horn with functional endometrium must be excised — pregnancy can occur there through transperitoneal sperm migration and rupture in the second trimester with catastrophic bleeding
• Order renal imaging, especially for unicornuate, didelphys and any obstructive anomaly — unilateral renal agenesis is the classic association. Symmetric anomalies (septate, arcuate) carry little renal risk
• The arcuate uterus is a normal variant. Do not operate on it
• Two cervices on speculum examination means didelphys — and prompts a look for a longitudinal vaginal septum and a check of the kidneys
• Uterine anomalies do not cause primary amenorrhea. Primary amenorrhea means agenesis or obstruction, not a misshapen uterus
• Septum resection is offered, not assumed: observational data favor it, but the randomized TRUST trial did not show a live-birth benefit. Counsel accordingly
• Vaginal delivery is fine after hysteroscopic metroplasty; cesarean is indicated after abdominal metroplasty because of the myometrial scar
• Cerclage for standard indications only — the anomaly alone is not an indication; serial cervical length surveillance is
• DES exposure in utero gives a T-shaped cavity with a cervical hood, and is the only in-utero-acquired uterine anomaly
• The embryologic sequence is develop → fuse → resorb: failure to develop gives agenesis or unicornuate, failure to fuse gives didelphys or bicornuate, failure to resorb gives septate or arcuate