BackgroundHigh discrepancy between current classifications was observed in the definition of uterine septa, especially for indentation lengths >5 <10mm.ObjectivesTo assess the discrepancy between current classifications in the diagnoses of septate uterus and to correlate them with reproductive outcomes; to detect 3D transvaginal ultrasound (TVS) additional measurements, which can better correlate small indentation lengths >5 <10mm to reproductive failures.Materials and MethodsObservational study enrolling 664 women of reproductive age with 3D ultrasound diagnosis of an indentation length ≥3mm. For each patient a detailed reproductive history was taken before performing 3D transvaginal examination. Patients with previous uterine surgery or metroplasty were excluded.Main outcome measure(s)Indentation lengths >5 <10mm showed high discrepancy in the diagnosis of uterine septum between different classifications. For these small indentations additional 3D measurements (indentation angle, septal width and septal length/ fundal myometrial thickness (L/M) ratio) were correlated to infertility and recurrent miscarriage.ResultsAmong the cohort, 215 patients showed an indentation length >5 <10mm; 136 tried to conceive: 69 (51%) were infertile, 38 (28%) had recurrent miscarriages (≥2) and 5 (4%) had at least one delivery. Recurrent miscarriage significantly correlated to an indentation angle >134°; whereas infertility to an indentation width <32mm and a L/M ratio >75%.ConclusionsWide discrepancies between different classifications are more evident in indentation lengths >5 <10mm. Additional measurements on 3D coronal section may help to evaluate the risk of infertility or recurrent miscarriage.What is new?Additional 3D TVS measurements, beyond septal lengths, in particular for small fundal indentation, may help in predicting the risk of developing adverse reproductive outcomes.