Fig. 6  (a–d) A 17-year-old female patient. Transvaginal US (a) depicts two endometrial cavities (arrows) in the coronal plane consisting of a hyperechogenic endometrial stripe and a hypoechogenic myometrial layer. An intervening external fundal cleft was visible during the examination by interactively turning the image plane (arrowheads). The highly suspected presence of a bicornuate uterus was

confirmed by MR with a coronal T2-weighted image (b) showing two uterine cavities (arrows) separated by a deep external fundal cleft (arrowheads). In the same image plane, the separation of both cavities (arrows) are visible to the level of the internal cervical os (c) before they conjoin to one single cervix (d) as shown in a T2-weighted axial image (bicornuate, unicollis uterus)

2.2.5\ Class V Anomalies: Septate Uterus

Definition: Septate uterus is the result of partial or complete nonregression of the midline ­uterovaginal septum (Pellerito et al. 1992; Carrington et al. 1990; Fedele and Bianchi 1995; Fedele et al. 1996; Reuter et al. 1989; Valle 1996) (Fig. 6). 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 (Homer et al. 2000; Carrington et al. 1990) (Fig. 7).

Septate uterus is the most common Müllerian 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 therefore – empirically invented 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

Fig. 7  (a, b). A 21-year-old female patient with infertility. (a) Coronal T2-weighted MR image demonstrates an uterine contour with mild external fundal cleft (arrowhead), as well as a hypointense thin septum dividing the

endometrial cavity at the level of the uterine body and cervix (arrows). (b) Axial T2-weighted image shows the extension of the longitudinal septum to the external cervix os

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.

2.2.6\ Class VI Anomalies: Arcuate Uterus

Definition: 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 (Buttram and Gibbons 1979) (Fig. 8).

Differentiation from bicornuate uterus is based on the complete fundal unification; however, a partial septate uterus with a broad-based muscular septum 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. Reports finding a higher risk of secondterm miscarriage mostly used suboptimal diagnostic tests like two-dimensional ultrasound or HSG, while studies relying upon more accurate methods­ could not reproduce these results (Chan et al. 2011a). Fact is that MRI may detect this abnormality, but typically, it is not clinically relevant because arcuate uterus has no proven significant negative effects on pregnancy outcome.

2.2.7\ Class VII Anomalies

DES-exposed uterus. DES (synthetic estrogen, diethylstilbestrol, 1948–1971) may induce abnormal myometrial hypertrophy in the fetal uterus forming small T-shaped endometrial cavities (Rennell 1979), as well as increase the risk of developing a clear cell carcinoma of the vagina (Herbst et al. 1971; Hatch et al. 1998). The characteristic uterine abnormalities must be categorized in the group of complex uterine anomalies and may occur with or without the exposure of DES.