Three-dimensional (3D) ultrasound acquires a volume of data rather than a single 2D plane, allowing the image to be reconstructed and viewed in any orientation including multiplanar reconstruction (MPR) and rendered surface views. In obstetrics, 3D is used to evaluate fetal facial anatomy, neural tube defects, and limb abnormalities. In gynecology, 3D allows coronal views of the uterine cavity — not obtainable with 2D — for evaluating Müllerian anomalies and IUD position. 4D ultrasound adds real-time display of the 3D volume.