Dysgenesis (segmental agenesis and variable hypoplasia) of the Mullerian ducts (uterus and upper 2/3 of the vagina) (Fig. 1.1). Mayer-Rokitansky-Küster-Hauser syndrome is the most common form of Class I anomaly and includes agenesis of uterus and vagina [5, 9, 10].

Unicornuate uterus is the result of partial or complete hypoplasia of one Mullerian duct (Fig. 1.1) [11]. Unicornuate uterus may be isolated (35 %) or associated with a contralateral rudimentary horn. The rudimentary horn presents with or without communication to the endometrial cavity and may be associated with or without endometrium, which is also called no cavity rudimentary horn. In patients with cavity non-communicating rudimentary horn, dysmenorrhea and hematometra may occur. Surgical resection either to relieve symptomatic pain or to reduce the risk of potential ectopic pregnancy is justified. As with every obstructed system, the risk of endometriosis is also increased with a non-communicating rudimentary horn. Renal malformations are common with unicornuate uterus and occur mostly on the same side as the rudimentary horn could be found [5].

Uterus didelphys is a result of complete non-fusion of the Mullerian ducts forming a complete uterine duplication with no communication between each other (Fig. 1.1).

Uterus didelphys may be associated with a longitudinal (75 %) or, more rarely, with a transverse vaginal septum, the latter causing obstructive hematometrocolpos.

Endometriosis as a result of retrograde menstruation may also occur in these conditions. A nonobstructive uterus didelphys is usually asymptomatic [5].

Bicornuate uterus is the result of incomplete fusion of the cranial parts of the Mullerian ducts (Fig. 1.1) [5–9, 12]. Two uterine cavities with normal zonal anatomy can be depicted. The leading imaging feature is a fundal cleft greater than 1 cm of the external uterine contour that helps to distinguish bicornuate uterus from septate uterus.

Extension of the intervening fundal cleft to the internal cervical os characterizes the complete bicornuate uterus with a single cervix (bicornuate, unicollis uterus), whereas variants of partial bicornuate uterus exist if the cleft is of variable length. Bicornuate uterus may be associated with a duplicated cervix (bicornuate bicollis uterus), as well as with a longitudinal vaginal septum that coexists in up to 25 % of bicornuate uterus. Nevertheless, a degree of communication is always present between both uterine cavities.

Still controversial is the need for surgical intervention and this is probably only necessary in specific cases.

A higher rate of cervical incompetence seems to be associated with bicornuate uterus [13].

Septate uterus is a result of partial or complete non-regression or the midline uterovaginal septum (Fig. 1.1) [12, 13]. The main imaging feature is that the external contour of the uterine fundus may be either convex or mildly concave (<1 cm) and not with a cleft greater than 1 cm, the latter defining a bicornuate or didelphic uterus.

Septate uterus is the most common Mullerian duct anomaly and is unfortunately associated with the poorest reproductive outcome. Because of different treatment options, septate uterus must be differentiated from bicornuate and didelphic uterus. A widely accepted definition – empirically established during laparoscopy procedures – states that a uterus is septate if the outer contour of the uterine fundus is only mildly concave in the presence of a septum. The cutoff of concavity is 1 cm; deeper concavity is associated with bicornuate uterus and uterus didelphys.

In a complete septate uterus, the septum extends to the external cervical os. In 25 % of septate uteri, the septum extends even further into the upper part of the vagina.

Obstetric outcome seems not to be correlated with the length of the septum. The septum may be composed of muscle or fibrous tissue and is not a reliable means of distinguishing septate and bicornuate uteri.

Resection of the septum by hysteroscopic metroplasty is indicated and may improve the reproductive outcome significantly [5].

Arcuate uterus is the result of a near-complete regression of the uterovaginal septum forming a mild and broad saddle-shaped indentation of the fundal endometrium (Fig. 1.1).

Differentiation from bicornuate uterus is based on the complete fundal unification; however, a broad-based septate uterus is difficult to distinguish from an arcuate uterus. There is much controversy as to whether an arcuate uterus should be considered a real anomaly or an anatomic variant. MRI may detect this abnormality, but, typically, it is not clinically significant because arcuate uterus has no significant negative effects on pregnancy outcome [5].

DES-exposed uterus (Fig. 1.1). DES (synthetic estrogen, diethylstilbestrol, 1948–1971) may induce abnormal myometrial hypertrophy in the fetal uterus forming small T-shaped endometrial cavities, as well as increase the risk of developing a clear cell carcinoma of the vagina.

The characteristic uterine abnormalities must be categorized in the group of complex uterine anomalies and may occur with or without the exposure of DES [5–10, 12–14].

Diagnosis of uterine anomalies should be based on diagnostic modalities that could determine the anatomical status of the female genital tract on an objective way.

The ideal diagnostic method should provide objective and measurable information on the anatomical status of the uterus in a noninvasive way [1–10, 12–15].

The available diagnostic methods that can be used in the investigation of the patient are as follows.

Once an MDA is suggested based on evidence from the patient history and physical examination, the next diagnostic step includes different imaging work-ups in order to detect and specify MDA and to guide further treatment options.

Before US and MRI were capable of visualizing MDA with a high accuracy, imaging of MDA was limited to hysterosalpingography (HSG). Since the diagnostic imaging properties of MDA include mainly the configuration of the endometrial cavity and the external uterine contour, HSG is able to depict only certain types of MDA, whereas it fails in other cases and stays nonspecific for precise diagnosis, the latter mainly due to the lack of the visualization of the outer uterine contour. Because of this drawback, HSG did not provide diagnoses with high degrees of confidence, and US and MRI soon began to play a larger role in assessment and treatment of patients. As HSG provides, besides the morphological, also the functional information of tubal patency, it is still used in the primary imaging work-up in case of infertility clarification.

Nowadays, the first imaging modalities in the MDA assessment include pelvic US – transabdominal US (performed with a 2.5- to 5-MHz probe, for evaluation of the entire abdomen, especially for associated renal malformations) and transvaginal US (performed with a 5- to 8-MHz endovaginal probe, for better delineation of the uterus, vagina, and ovaries) – and MRI. Newer techniques, such as 3D US, even further improved the imaging diagnostics by giving better information about the external contour of the uterus and its volume [5, 16, 17].

Magnetic resonance imaging (MRI) is today considered standard in the evaluation of MDA and accepted as the leading imaging modality for further surgical planning. MRI provides high-resolution images of the entire uterine anatomy (internal and external contour), as well as of secondary findings like renal malformations.

Among the three major imaging methods, MRI has the best accuracy in the evaluation of uterine anomalies (up to 100 % has been reported) [5, 8, 16, 17, 23, 24].