Objective: To define the added value of three-dimensional transvaginal ultrasound (3D TVUS) and three-dimensional gel infusion sonography (3D GIS) in the diagnostic work-up of patients with a uterine septum by determining the sensitivity, specificity and interrater reproducibility. 
Design: Prospective multicentre cohort study.
Patients: Patients aged 18-45 years with suspicion of a uterine septum on two-dimensional transvaginal ultrasound (2D TVUS), without prior septum surgery.
Interventions: Three-dimensional transvaginal ultrasound (3D TVUS) and three-dimensional gel infusion sonography (3D GIS). 
Comparison: Magnetic resonance imaging (MRI) of the pelvis.
Primary outcome measure: The sensitivity of 3D TVUS for the diagnosis of a uterine septum compared with MRI.
Secondary outcome measures:
The specificity of 3D TVUS for the diagnosis of a uterine septum compared with MRI.
The sensitivity and specificity of 3D GIS for the diagnosis of a uterine septum compared with MRI.
The interrater reproducibility of 3D TVUS and 3D GIS for the diagnosis of a uterine septum. 
Patient experience of 3D TVUS, 3D GIS and MRI. 
Cost-effectiveness of 3D TVUS and 3D GIS for the diagnosis of a uterine septum compared with MRI.
Planned sample size: 145 patients.