Technological advances have markedly improved the quality of ultrasound equipment in the last decades. Ultrasound is an essential non-invasive diagnostic tool in many medical fields, particularly in fetal medicine. In this context, the advent of three-dimensional ultrasound has importance as it provides a new technique for evaluating growth and development of the fetus, also allowing diagnosis of fetal structural anomalies with high accuracy, after initially being suspected using two-dimensional ultrasound. 3D ultrasound is therefore an adjunctive method to two-dimensional conventional ultrasound.

Three-dimensional ultrasound is a relatively new diagnostic imaging method similar to computed tomography and magnetic resonance imaging, in which the transducer captures multiple sequential two-dimensional images, known as the tomograms, and turns them into a set of three-dimensional data composed of cubic units called “voxels” (volume elements). After volume scan with computer processing, three orthogonal section images are displayed at the same time: longitudinal, coronal, and transverse. After that, it is possible to obtain other multiple views by moving and rotating each of the planes up to 360°. Therefore, it allows views to be obtained that are initially not accessible to conventional ultrasound. The three-dimensional data can also be processed by surface mode, which is useful for surface delineation, especially when they are surrounded by fluid; maximum transparency mode highlights the densest echoes such as bones and the minimum transparency mode highlights hypoechoic structures such as vessels and cysts. The X-ray mode is the projection of gray tonalities on a particular plane inside the whole volume.

Three dimensional ultrasound surface mode is recommended, for example, to study fetal face morphology, to investigate markers of certain syndromes such as facial dysmorphisms, cleft lip, cleft lip and palate, micro- and retrognathia, and the presence or absence of a nasal bone. It is also useful for evaluating fetal ear, its morphology, orientation, and location in the skull. Regarding the central nervous system, one of the advantages of 3D ultrasound is to obtain the median sagittal plane to identify the corpus callosum, as it can be difficult to see on two-dimensional ultrasound. The association of 3D color or power Doppler to evaluate vessels is useful for the diagnosis of vascular abnormalities. Maximum transparency and X-rays are the chosen modes for extremities and skeleton assessment because these bony structures are visualized.

Other applications of 3D ultrasound consist in the functional assessment of fetal organs. Chest pulmonary hypoplasia and lung malformations can be evaluated by calculation of volume using the rotational technique (or Virtual Organ Computer-aided Analysis, VOCAL_TM). The measurement of placental volume is being studied to predict preeclampsia, fetal growth restriction, and placental insufficiency. 3D ultrasound can also be used to evaluate the vascularity of one organ or fetal structure by combining the 3D ultrasound, power Doppler, and VOCAL_TM.

Four-dimensional ultrasound consists in the visualization of a three-dimensional image in “real time,” i.e., sequential three-dimensional images are displayed while the object under study is moving. However, the image displayed is not exactly in real time because the device requires a certain time to process the 3D image. 3D and 4D ultrasound have been widely used in clinical practice to show the fetus and its behavior, facilitating the understanding of the ultrasound image by laypersons, promoting and strengthening the bond of the fetus with the parents and family. It is possible to visualize movements such as facial expressions, opening and closing mouth and eyes, hands, and legs, etc. In invasive procedures such as cordocentesis, 3D ultrasound allows the identification of the object of interest in three orthogonal planes simultaneously on the same screen, facilitating target reaching and, in cases selected for fetoscopy, the device can be monitored in the amniotic cavity by real-time 4D ultrasound.

Three-dimensional ultrasound does not replace conventional ultrasound, but it is considered as an additional and complementary method to 2D ultrasound.