How do Müllerian anomalies arise from duct development?
The female genital tract develops from two paired duct systems between roughly the sixth and twentieth weeks of gestation. The mesonephric (Wolffian) duct appears first, and the paramesonephric (Müllerian) duct forms in the seventh week as an invagination of coelomic epithelium. The paired Müllerian ducts grow caudally beside the Wolffian ducts, cross them, and meet in the midline to form the uterus, cervix and upper vagina. The lower vagina joins this system from the urogenital sinus.
Development is usually taught in three phases, and almost every anomaly can be mapped to one of them. Failure of formation gives agenesis, hypoplasia or a unicornuate uterus. Failure of fusion of the two ducts gives a didelphys or bicornuate uterus. Failure of resorption of the midline septum, which normally disappears by about week 20, gives a septate or arcuate uterus.
| Developmental step | What goes wrong | Resulting anomaly |
|---|---|---|
| Formation (organogenesis) | One or both ducts fail to develop | Müllerian agenesis (MRKH), unicornuate uterus with or without a rudimentary horn |
| Fusion (lateral) | The two ducts do not unite, completely or partly | Uterus didelphys (complete), bicornuate uterus (partial) |
| Septal resorption | The midline septum persists after fusion | Septate uterus (complete or partial), arcuate uterus (minimal) |
How are Müllerian anomalies classified (ASRM 2021 and ESHRE/ESGE)?
The American Society for Reproductive Medicine (ASRM) Müllerian Anomalies Classification 2021 (MAC2021) replaced older numbered classes with descriptive categories that apply across all levels of care. It recognises nine categories: Müllerian agenesis, cervical agenesis, unicornuate uterus, uterus didelphys, bicornuate uterus, septate uterus, longitudinal vaginal septum, transverse vaginal septum and complex anomalies.
The ESHRE/ESGE system (2013) is anatomy-based. Uterine main classes are U0 normal, U1 dysmorphic, U2 septate, U3 bicorporeal, U4 hemi-uterus, U5 aplastic and U6 unclassified. Cervical and vaginal anomalies are coded independently in supplementary sub-classes, so a single patient receives a combined uterine, cervical and vaginal code. What was traditionally called a didelphys uterus falls under complete bicorporeal uterus (U3b) with a double cervix.
| Traditional name | ASRM MAC2021 category | ESHRE/ESGE class |
|---|---|---|
| Normal uterus | — | U0 |
| T-shaped / infantile uterus | Complex or other | U1 dysmorphic |
| Septate (including arcuate end of spectrum) | Septate uterus | U2 septate |
| Bicornuate | Bicornuate uterus | U3 bicorporeal |
| Didelphys | Uterus didelphys | U3b with double cervix |
| Unicornuate ± rudimentary horn | Unicornuate uterus | U4 hemi-uterus |
| MRKH / uterine aplasia | Müllerian agenesis | U5 aplastic |
What defines a septate or arcuate uterus, and how is it treated?
A septate uterus results from normal fusion but failed resorption of the midline septum. The septum may be partial, extending down from the fundus without reaching the internal os, or complete. It is the most commonly encountered Müllerian anomaly in clinical practice, accounting for about 55% of diagnoses, with a prevalence estimated at up to 1.5% of reproductive-age females. Many affected people are asymptomatic.
Definitions differ between societies, which is why reported prevalence varies. ASRM defines septate uterus as an indentation depth greater than 15 mm with an indentation angle less than 90°. ESHRE uses an indentation-to-wall-thickness ratio greater than 50%, and the CUME group suggests an indentation depth of at least 1 cm.
If resorption stops very late, only a slight midline indentation remains — the arcuate or heart-shaped cavity, which appears to have no known clinical relevance. In the general population the arcuate uterus is the most frequently found variant, whereas among people with infertility or miscarriage the septate uterus is the most frequent.
- Treatment: the standard intervention for a symptomatic septum is hysteroscopic septal resection (metroplasty) — the outer uterine wall is normal, so no abdominal unification is needed.
- Evidence: observational studies suggest resection can improve pregnancy rates and outcomes after miscarriage or preterm birth, but evidence that it improves reproductive outcomes is described as equivocal.
- Old open operations: Jones and Tompkins abdominal metroplasties are now rarely performed because of their surgical and obstetric risk.
How do bicornuate uterus and uterus didelphys differ?
A bicornuate uterus results from incomplete fusion of the Müllerian ducts, giving two distinct cavities and a heart-shaped outline instead of the usual pear shape. In bicornuate unicollis the upper segments fail to fuse while the lower segment and cervix fuse normally, giving one cervix. In bicornuate bicollis both upper and lower segments fail to fuse, giving two cavities and two cervices.
Uterus didelphys is the complete failure of fusion: two separate uterine bodies, each with its own cervix, often with a longitudinal vaginal septum. On imaging, the presence of soft tissue between the two cavities favours a bicornuate uterus over didelphys when a vaginal septum is present in both.
OHVIRA syndrome (obstructed hemivagina and ipsilateral renal agenesis), also called Herlyn–Werner–Wunderlich syndrome, is the classic didelphys variant. One hemivagina is blocked, so menstrual blood collects behind the obstruction, and the kidney on the same side is absent. In one Indian tertiary-centre series of 25 patients, the mean age at presentation was 17.7 years, dysmenorrhoea (72%) and pelvic pain (60%) were the commonest symptoms, haematocolpos was the commonest complication, and vaginal septal resection was the primary treatment.
A bicornuate uterus that causes recurrent pregnancy loss or preterm labour may be unified by Strassman metroplasty, first described in 1907: a transverse fundal incision avoiding the uterotubal junctions, excision of the intervening tissue and closure into a single cavity.
Why is a rudimentary horn of a unicornuate uterus dangerous?
A unicornuate uterus (ESHRE U4, hemi-uterus) is a formation defect in which only one Müllerian duct develops normally. The contralateral side may be absent or form a rudimentary horn, which can be with or without a cavity and communicating or non-communicating with the main cavity. ESHRE uses the presence of a cavity in the horn for sub-classification because a functional cavity causes cyclic pain or haematometra and needs treatment.
A pregnancy implanted in the rudimentary horn behaves like an ectopic pregnancy. In a 2026 systematic review of 190 published cases, 85.8% were in non-communicating horns. Rupture occurred in 35.8%, and rupture happened earlier in non-communicating horns (about 18.7 weeks on average) than in communicating ones. Diagnosis is often late, which is why horns with a cavity are usually excised.
What is MRKH syndrome and how does it present?
Mayer–Rokitansky–Küster–Hauser (MRKH) syndrome, or Müllerian aplasia, is congenital aplasia of the uterus and upper vagina in a person with a normal 46,XX karyotype and normal secondary sexual characteristics. Ovarian function is normal, so thelarche and pubarche occur on time. The estimated prevalence is about 1 in 5000 live female births.
The usual presentation is primary amenorrhoea in adolescence. MRKH accounts for about 16% of primary amenorrhoea and is considered the second most common cause after ovarian failure — a key point in the amenorrhoea workup. Examination shows normal breasts and pubic hair with a short, blind-ending vagina.
| Feature | MRKH type I | MRKH type II | Complete androgen insensitivity |
|---|---|---|---|
| Karyotype | 46,XX | 46,XX | 46,XY |
| Uterus and upper vagina | Absent | Absent | Absent |
| Gonads | Normal ovaries | Normal ovaries | Testes |
| Extragenital anomalies | None (isolated) | Renal, skeletal, ear or cardiac; includes MURCS association | — |
| Pubic and axillary hair | Normal | Normal | Sparse or absent |
- MURCS association: Müllerian duct aplasia, renal aplasia and cervicothoracic somite dysplasia — part of type II.
- Vaginal agenesis: non-invasive progressive self-dilation (Frank's method) is recommended first-line; surgical vaginoplasty (for example traction-based techniques) is the alternative.
- Fertility: absolute uterine factor infertility; options are gestational surrogacy where permitted or uterus transplantation, with the first live birth after transplantation reported in Gothenburg in 2014.
Which imaging best diagnoses Müllerian anomalies?
Hysterosalpingography (HSG) is often the first test in infertility because it shows cavity shape and tubal patency, but it sees only the inside of the uterus. It therefore cannot reliably distinguish a septate from a bicornuate uterus, and a non-communicating horn will not fill with contrast.
3D transvaginal ultrasound shows both the cavity and the external fundal contour in the coronal plane. Combined hysteroscopy and laparoscopy has the same diagnostic yield for a septum as 3D ultrasound, so the less invasive 3D scan is preferred. MRI is used when ultrasound is inconclusive, for complex or obstructive anomalies, and to differentiate fusion from resorption anomalies.
| Modality | Strength | Limitation |
|---|---|---|
| HSG | Cavity outline and tubal patency | No view of the external contour |
| 2D ultrasound | Widely available first look | Coronal fundal view is limited |
| 3D transvaginal ultrasound | Cavity plus external contour in one coronal view | Operator dependent |
| MRI | Complex, obstructed and rudimentary-horn anomalies | Cost and access |
| Hysteroscopy with laparoscopy | Direct inside and outside view; allows treatment | Invasive, needs anaesthesia |
A classic measurement for bicornuate uterus (Troiano and McCarthy) draws a line between the interstitial portions of the tubes and measures the fundal indentation perpendicular to it: an indentation greater than 10 mm supports bicornuate uterus, while less than 10 mm suggests an arcuate or subseptate uterus.
What obstetric problems and renal anomalies accompany them?
Uterine anomalies are more common in people with reproductive problems. A review of 94 observational studies found a prevalence of 8.0% in infertility, 13.3% after miscarriage and 24.5% when both were present. Reported adverse outcomes include infertility, recurrent pregnancy loss, preterm labour, fetal growth restriction, malpresentation, placental abruption and caesarean delivery.
- Bicornuate uterus: preterm labour is the most common complication; a cervical length below 25 mm on transvaginal scan carries about a 13-fold higher risk of preterm delivery, and cervical cerclage is used when the cervix is significantly short.
- Septate uterus: the most common adverse outcomes are infertility and preterm delivery, though many patients have none.
- Rudimentary horn: ectopic-type pregnancy with risk of rupture.
- Postpartum haemorrhage has also been reported after pregnancy in a bicornuate uterus.
Because the Müllerian and mesonephric ducts develop together, urinary tract anomalies coexist in up to 40% of patients. The most common pairing is renal agenesis with uterus didelphys, but renal anomalies also occur with bicornuate uterus and an ectopic ureter may be found. They are thought to be less frequent with resorption defects such as a septum than with fusion defects.