A) Didelphys uterus INCORRECT
A didelphys uterus occurs as a result of complete nonfusion of the Müllerian duct, resulting in duplicated uterine horns, duplicated cervices, and occasionally the proximal vagina, which is often associated with a transverse or longitudinal vaginal septum.1,2 Fusion anomalies (didelphys and bicornuate) can be differentiated from resorption anomalies (septate and arcuate) on ultrasonography (US) by a deep fundal cleft (>1 cm).1 3D US is highly accurate in identifying Müllerian duct anomalies (MDAs), it correlates well with magnetic resonance imaging (MRI), and it is more cost effective and accessible.3 For the didelphys uterus, 3D US obtained coronal to the uterine fundus often demonstrates widely divergent uterine horns separated by a deep serosal fundal cleft (>1 cm) and noncommunicating endometrial cavities with 2 cervices.1 Identifying a duplicated vaginal canal can help differentiate a didelphys uterus from a bicornuate bicollis uterus.1
Didelphys uterus. Transabdominal US of the pelvis demonstrates 2 widely divergent uterine horns (long arrows) separated by echogenic fat (arrowhead).
B) Unicornuate uterus INCORRECT
A unicornuate uterus results from arrested development of one Müllerian duct and normal development of the other. There are 4 different subtypes: no rudimentary horn, rudimentary horn without endometrial cavity, rudimentary horn with communicating endometrial cavity, and rudimentary horn with noncommunicating uterine cavity.1 Renal anomalies, especially renal agenesis when present, are usually ipsilateral to the rudimentary horn.1 On US, unicornuate uterus appears as a lobular oblong (banana-shaped) structure away from the midline. The rudimentary horn is often difficult to identify and, when seen, the presence or absence of endometrium is an important finding.1
Unicornuate uterus. (A) 3D surface-rendered ultrasound of the uterus demonstrates an oblong curvilinear uterine horn to the right of midline (long arrow). (B) Transvaginal pelvic ultrasound image demonstrates a small noncommunicating rudimentary horn to the left of midline (small arrow) adjacent to the unicornuate uterus on the right (long arrow). No endometrium is imaged within this rudimentary horn.
C) Partial septate uterus CORRECT
A septate uterus is a result of a resorption anomaly. The septum may be partial (subseptate) or complete secondary to failure of resorption of the uterovaginal septum. A complete septum extends to the external cervical os and occasionally to the vagina.1 US demonstrates interruption of the myometrium by a hypoechoic fibrous septum and/or a muscular septum which is isoechoic to the myometrium and arises midline from the fundus.1 3D US often clearly delineates the septum and improves visualization of the external fundal contour.2 The external fundal contour in septate uterus is convex (little or no serosal fundal cleft) and the apex is greater than 5 mm above the interostial line compared with a bicornuate or didelphys uterus, where the apex is less than 5 mm from the interostial line.1,2 Additionally, the intercornual distance is less than 4 cm.4 A septate uterus with duplicated cervix can appear similar to a bicornuate bicollis uterus and can be correctly diagnosed by evaluating the external fundal contour.1
Partial septate uterus. (A) Transvaginal pelvic ultrasound (transverse image) demonstrates an isoechoic septum (long arrow) separating the 2 uterine cavities. (B) 3D surface-rendered image clearly delineates the midline septum (long arrow) arising from the fundus without extending to the cervix with the interostial distance measuring 2.4 cm and 2.6 cm deep from the tubal ostial line. Also seen is a convex external fundal contour (no fundal serosal cleft noted) (short arrow).
D) Arcuate uterus INCORRECT
The arcuate uterus also represents a resorption anomaly as a sequela of near-complete resorption of the uterovaginal septum.1 On 3D US, the fundal indentation in an arcuate uterus is well delineated and appears broad based with an obtuse angle at the most central point, which can help differentiate it from a septate uterus that has an acute angle.2,3 Both an arcuate and a septate uterus have a normal external (serosal) fundal contour.2,3 With an arcuate uterus, the length of the vertical measurement from the tubal ostia line, which is drawn horizontally between the tubal ostia to the fundal midline component of the endometrial cavity, is 1 cm or less. With a septate uterus, this vertical distance is greater than 1 cm.
Arcuate uterus. This 3D image of the uterus demonstrates a broad-based fundal indentation (long arrow) in the endometrial cavity, 8 mm deep at the most central point along with a convex external (serosal) fundal contour (short arrow) indicative of an arcuate uterus.