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Summary Table

Category	Details
Risk Factors	In utero exposure to diethylstilbestrol (DES), family history of Mullerian anomalies, associated renal anomalies (unilateral renal agenesis), skeletal anomalies
Etiology	Failure of normal Mullerian (paramesonephric) duct development during embryogenesis: agenesis, failed fusion, or failed septal resorption
Patient Presentation	Varies by type: primary amenorrhea (agenesis), recurrent pregnancy loss (septate uterus), preterm labor and malpresentation (unicornuate/bicornuate), dysmenorrhea with obstructed outflow
Classic Physical Exam	Normal secondary sexual characteristics with absent menses (Mayer-Rokitansky-Kuster-Hauser syndrome), longitudinal vaginal septum (didelphys), blind vaginal pouch, shortened or absent vagina
Key Diagnostic Results	Pelvic ultrasound (initial), MRI (gold standard for classification), 3D sonohysterography, hysterosalpingography for cavity assessment; normal 46,XX karyotype in Mullerian agenesis
Management	Septate uterus: hysteroscopic metroplasty (septum resection). Mullerian agenesis: vaginal dilators (first-line) or surgical neovagina (McIndoe procedure). Bicornuate/didelphys: generally no surgical correction unless severe obstetric complications. Unicornuate: monitor for ectopic pregnancy in rudimentary horn
Keywords	"Primary amenorrhea with normal secondary sexual characteristics," "recurrent second-trimester loss," "breech presentation," "blind vaginal pouch," "absent uterus with 46,XX karyotype," "longitudinal vaginal septum"

1. Pathophysiology

The uterus, fallopian tubes, cervix, and upper two-thirds of the vagina all originate from the paired Mullerian (paramesonephric) ducts. Normal development requires three sequential steps: duct formation (elongation), duct fusion (the two ducts merge in the midline to form a single uterine cavity), and septal resorption (the medial walls between the fused ducts dissolve to create one continuous cavity). A defect at any of these three steps produces a different category of anomaly.

Mullerian agenesis (Mayer-Rokitansky-Kuster-Hauser syndrome, or MRKH) results from complete failure of duct development. The ducts never form, so the uterus, cervix, and upper vagina are absent. Because the ovaries derive from a separate embryologic origin (the gonadal ridge, not the Mullerian system), ovarian function is entirely normal. This is why these patients have normal secondary sexual characteristics (breast development, axillary and pubic hair from normal estrogen and androgen production) but present with primary amenorrhea. The lower one-third of the vagina, which derives from the urogenital sinus rather than the Mullerian ducts, may be partially present as a blind-ending pouch.

Unicornuate uterus arises from failure of one Mullerian duct to develop while the other develops normally. The result is a single-horned, banana-shaped uterus with only one fallopian tube. In many cases, a rudimentary horn from the undeveloped side persists. If this horn contains functional endometrium but lacks communication with the main cavity or vagina, menstrual blood becomes trapped, causing cyclic pelvic pain in adolescence. A rudimentary horn also creates a risk of ectopic implantation, which can rupture catastrophically because the myometrial wall is thin and poorly distensible.

Uterus didelphys results from complete failure of fusion. Both Mullerian ducts develop fully but never merge, producing two entirely separate uterine bodies, two cervices, and often a longitudinal vaginal septum. Despite the dramatic anatomy, reproductive outcomes in didelphys are relatively better than in other anomalies because each hemi-uterus has a functional, well-vascularized myometrium.

Bicornuate uterus results from partial failure of fusion. The fundus remains divided into two horns that share a single cervix (bicornuate unicollis) or, rarely, two cervices (bicornuate bicollis). The external fundal contour shows a deep fundal cleft (>1 cm indentation), which is the key feature distinguishing it from a septate uterus on imaging. Pregnancy can occur, but the distorted cavity increases the risk of malpresentation (breech), preterm labor, and cervical incompetence.

Septate uterus results from failure of septal resorption after successful fusion. The external contour of the uterus is normal (smooth, convex fundus), but a fibrous or fibromuscular septum projects into the cavity, dividing it partially or completely. This is the most common Mullerian anomaly and, critically, the one most strongly associated with recurrent pregnancy loss, particularly in the first and second trimesters. The septum has a poor blood supply compared to normal myometrium, so embryos that implant on the septum receive inadequate perfusion, leading to implantation failure or placental insufficiency.

DES-related (T-shaped) uterus is caused by in utero exposure to diethylstilbestrol, a synthetic estrogen prescribed to pregnant women between the 1940s and 1970s to prevent miscarriage. The drug disrupted Mullerian duct development, producing a characteristic T-shaped uterine cavity with constriction bands, a hypoplastic uterus, and increased risk of clear cell adenocarcinoma of the vagina/cervix, ectopic pregnancy, and recurrent pregnancy loss.

A critical embryologic link that is tested repeatedly: the Mullerian and mesonephric (Wolffian) duct systems develop in close physical proximity, so Mullerian anomalies are frequently associated with renal anomalies (unilateral renal agenesis, pelvic kidney, horseshoe kidney). Any time a Mullerian anomaly is identified, renal imaging is indicated.

2. Classification and Clinical Manifestation

The American Fertility Society (now ASRM) classification organizes these anomalies by the embryologic step that failed:

Class	Anomaly	Embryologic Defect	Key Clinical Feature	Reproductive Impact
I	Mullerian agenesis (MRKH)	Failure of duct formation	Primary amenorrhea, absent uterus and upper vagina, normal 46,XX karyotype, normal secondary sexual characteristics	Infertility (no uterus); surrogacy or uterine transplant required for pregnancy
II	Unicornuate uterus	Unilateral failure of duct development	Banana-shaped single-horn uterus; may have a rudimentary horn	Ectopic pregnancy in rudimentary horn, preterm delivery, malpresentation
III	Uterus didelphys	Complete failure of fusion	Two uterine bodies, two cervices, longitudinal vaginal septum	Relatively better outcomes; preterm delivery, breech presentation
IV	Bicornuate uterus	Partial failure of fusion	Heart-shaped uterus with deep fundal cleft (>1 cm), single or double cervix	Preterm labor, malpresentation, cervical incompetence
V	Septate uterus	Failure of septal resorption	Normal external uterine contour with internal septum dividing the cavity	Highest rate of recurrent pregnancy loss among all Mullerian anomalies; most common anomaly overall
VI	Arcuate uterus	Mild/near-complete septal resorption	Minimal fundal indentation (<1 cm), nearly normal cavity	Generally considered a normal variant with minimal clinical impact
VII	DES-related (T-shaped)	In utero DES exposure	T-shaped cavity, hypoplastic uterus, cervical abnormalities	Ectopic pregnancy, recurrent pregnancy loss, clear cell adenocarcinoma risk

3. Diagnostic Workup

Test	Role	Key Findings
2D Pelvic ultrasound	Best initial test	Identifies absent uterus, duplicated structures, or abnormal fundal contour; limited ability to distinguish septate from bicornuate
3D Ultrasound / Sonohysterography	Improved cavity and fundal assessment	Saline infusion delineates the internal cavity; 3D reconstruction can visualize both the external contour and internal septum simultaneously
Pelvic MRI	Gold standard / most accurate test	Simultaneously evaluates the external uterine contour, internal cavity shape, and associated renal anatomy; definitively distinguishes septate (flat/convex fundus) from bicornuate (deep fundal cleft)
Hysterosalpingography (HSG)	Cavity and tubal assessment	Shows two endometrial cavities in septate, bicornuate, and didelphys, BUT cannot assess external fundal contour, so it cannot distinguish septate from bicornuate
Karyotype	Rule out androgen insensitivity syndrome	Normal 46,XX in MRKH; if karyotype returns 46,XY, the diagnosis shifts to androgen insensitivity syndrome
Renal ultrasound	Screen for associated anomalies	Indicated in every patient with a confirmed Mullerian anomaly due to shared embryologic origin of Mullerian and mesonephric systems
Hysteroscopy/Laparoscopy	Direct visualization	Hysteroscopy assesses the internal cavity and septum; laparoscopy evaluates the external fundal contour; largely replaced by MRI for diagnostic purposes but remains relevant for surgical planning

The diagnostic workup on exams follows a predictable pattern. The vignette begins with a clinical scenario (primary amenorrhea, recurrent pregnancy loss, or incidental finding on imaging) and asks for the next step. Pelvic ultrasound is always the best initial test because it is noninvasive, widely available, and sufficient to detect the presence or absence of the uterus and obvious structural duplications.

The critical diagnostic distinction tested on exams is septate versus bicornuate uterus, because the management differs completely. HSG will show a divided cavity in both conditions but cannot visualize the outside of the uterus, making it unable to distinguish the two. The exam will often present an HSG result showing "two endometrial cavities" and then ask for the next step. The answer is MRI, which shows both the internal cavity and the external fundal contour in a single study. A septate uterus has a normal convex or flat external fundal contour with an internal septum, while a bicornuate uterus has a deep external fundal cleft exceeding 1 cm.

For a patient presenting with primary amenorrhea, normal breast development, and absent uterus on ultrasound, the next step is a karyotype. This differentiates MRKH (46,XX) from complete androgen insensitivity syndrome (46,XY), which also presents with primary amenorrhea, absent uterus, and female phenotype but has a fundamentally different etiology (testosterone receptor insensitivity in a genetically male individual with intra-abdominal testes).

4. Management and Treatment

Anomaly	Primary Management	Details
Septate uterus	Hysteroscopic metroplasty (septum resection)	Outpatient hysteroscopic procedure to resect the septum; indicated for recurrent pregnancy loss or infertility; this is the only Mullerian anomaly with a straightforward surgical correction
MRKH (Mullerian agenesis)	Progressive vaginal dilators (first-line)	Frank dilator method: patient applies graduated dilators to the vaginal dimple for 20 to 30 minutes daily over several months to create a functional neovagina; success rate exceeds 90%
MRKH (if dilators fail)	McIndoe surgical vaginoplasty	Surgical creation of a neovaginal canal using a split-thickness skin graft over a mold; reserved for patients who fail or cannot tolerate dilator therapy
Bicornuate uterus	Expectant management (most cases)	No surgical correction unless severe, recurrent obstetric complications; Strassman metroplasty (open abdominal procedure to unify the two horns) is rarely performed
Uterus didelphys	Expectant management	Resection of vaginal septum if it causes obstructed outflow or dyspareunia; uterine unification almost never indicated
Unicornuate with rudimentary horn	Laparoscopic excision of rudimentary horn	Indicated if the horn contains functional endometrium (causing pain or ectopic pregnancy risk); prophylactic removal is recommended
DES-related uterus	Surveillance	No corrective procedure available; close obstetric monitoring for ectopic pregnancy and cervical incompetence; screen for clear cell adenocarcinoma of vagina/cervix
All anomalies during pregnancy	High-risk obstetric monitoring	Serial cervical length measurements, cerclage if cervical incompetence develops, planned delivery for malpresentation

The management decision tree is built around one central question: is the anomaly surgically correctable, and will correction improve reproductive outcomes?

The septate uterus is the most important anomaly to recognize because it is both the most common and the most treatable. Hysteroscopic septum resection is a minimally invasive procedure performed through the cervix with no abdominal incision, and it substantially reduces pregnancy loss rates. The exam frequently tests whether students know that septate uterus is the appropriate indication for hysteroscopic metroplasty, while bicornuate uterus is not (because cutting a bicornuate septum would enter the myometrium and risk uterine rupture).

For MRKH, the first-line treatment is nonsurgical: progressive vaginal dilators. This is a commonly tested point because students often jump to surgery. Dilator therapy is effective, carries no surgical risk, and is recommended by guidelines as the initial approach. Surgery (McIndoe procedure) is reserved for failure of conservative management.

For unicornuate uterus with a non-communicating rudimentary horn containing functional endometrium, excision of the horn is indicated to prevent hematometra (trapped menstrual blood causing cyclic pain) and to eliminate the risk of ectopic implantation. If a pregnancy does implant in a rudimentary horn, the thin myometrial wall cannot support growth, and rupture with life-threatening hemorrhage can occur in the second trimester.

During pregnancy in any patient with a known Mullerian anomaly, heightened surveillance is required. The reduced or abnormally shaped uterine cavity limits fetal mobility, increasing the rate of breech presentation and preterm delivery. Serial cervical length assessment by transvaginal ultrasound is performed, and cerclage placement may be necessary if progressive cervical shortening is detected.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Looks Similar	Key Discriminator
Complete androgen insensitivity syndrome (CAIS)	Primary amenorrhea, absent uterus, female phenotype, normal breast development	Karyotype is 46,XY (not 46,XX); patient has intra-abdominal testes (not ovaries); absent pubic and axillary hair due to androgen receptor insensitivity; testosterone levels are in the male range
Imperforate hymen	Primary amenorrhea in an adolescent, possible pelvic mass	Uterus and vagina are present and normally formed; bulging bluish membrane at the introitus on exam; hematocolpos (distended vagina filled with blood) on ultrasound; cyclic pain is present
Transverse vaginal septum	Primary amenorrhea, obstructed outflow, cyclic pelvic pain	Uterus is normally formed; obstruction is a transverse band within the vagina (not a longitudinal septum or absent vagina); diagnosed by MRI showing a septum with hematometra/hematocolpos above it
Asherman syndrome	Recurrent pregnancy loss, abnormal uterine cavity on imaging	Acquired condition (not congenital); history of prior uterine instrumentation (D&C, endometrial ablation); intrauterine adhesions (synechiae) seen on hysteroscopy; normal external uterine contour
Bicornuate vs. Septate uterus	Both show divided cavity on HSG; both associated with pregnancy loss	External fundal contour on MRI: septate has a normal/flat/convex fundus, bicornuate has a deep cleft >1 cm; this distinction dictates management (hysteroscopic resection vs. expectant)
Turner syndrome (45,X)	Primary amenorrhea	Absent secondary sexual characteristics (no breast development) due to gonadal dysgenesis (streak gonads); short stature, webbed neck, shield chest; karyotype is 45,X; uterus is present but infantile

6. Traps and High-Yield Pearls

The most commonly tested trap on this topic is the septate versus bicornuate distinction. Test writers know that students see "divided uterine cavity" and stop thinking. The vignette will provide an HSG result showing two hemicavities and then ask for the next step or the diagnosis. Selecting "bicornuate uterus" based on HSG alone is incorrect because HSG cannot see the external fundal shape. The correct answer is to order an MRI, and the correct distinction is: smooth external fundus equals septate, deep cleft equals bicornuate. This matters because only the septate uterus is treated with hysteroscopic resection.

The second major trap involves the primary amenorrhea workup. A patient with normal breast development, absent uterus, and 46,XX karyotype is MRKH. A patient with the same phenotype but 46,XY karyotype is complete androgen insensitivity syndrome. The distinguishing feature beyond karyotype is axillary and pubic hair: present in MRKH (functioning androgen receptors) and absent or sparse in CAIS (non-functioning androgen receptors). Test writers exploit this by presenting the phenotype and asking for either the karyotype or the next diagnostic step.

A third tested pearl: when a Mullerian anomaly is diagnosed, always screen for renal anomalies. This is a "freebie" association question, and the correct answer is renal ultrasound. The shared embryologic proximity of the Mullerian and Wolffian duct systems makes ipsilateral renal agenesis the most commonly associated finding, particularly with unicornuate and didelphys anomalies.

Finally, remember that the septate uterus is the most common Mullerian anomaly and carries the worst reproductive prognosis among surgically correctable anomalies, but it is also the easiest to fix. The core competency being tested is the student's ability to identify which anomaly benefits from intervention and to sequence the diagnostic workup correctly: ultrasound first, MRI to classify, then appropriate management based on the confirmed anatomy.

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