The vertebrate body has external bilateral symmetry, but an internal asymmetry of organs along the left–right axis, which is handed or directional, known as situs.
Determining the fetal sidedness and situs is mandatory in prenatal diagnosis and the first step of every fetal ultrasonic scan. It is especially important when diagnosing congenital heart disease and cardiovascular anomalies.
The currently published methods to distinguish fetal left from right side on the ultrasound examination rely on several parameters, including maternal position, fetal position, transducer orientation, display of the figure on the video screen, and observer skill. Thus, they are highly operator dependent complicated and difficult to record or document.
Since most fetuses scanned have normal situs, and abnormalities are rare, a simple method is needed to verify the normal fetuses and reliably diagnose the abnormal ones. The technique should be standard, easy to learn and perform, time efficient, one which can be performed reliably at every fetal scan. Operator dependency should be minimized.
We have devised a simple method based on scanning the fetus in two orthogonal planes –namely a long axis reference view and a short axis perpendicular view by which the fetal sidedness is reliably determined. Once done, demonstrating the position and direction of the cardiac apex and position of the gastric bubble determines the situs.
Because abnormal situses are rare, we chose to validate our methods using fetal models representing four possible situses, and since fetal models with heterotaxy are unavailable, we constructed four simple fetal models. Each model represents a fetus with a different visceral situs inside a womb.
We describe the rationale of our method, the technical details and show how different fetal situses are diagnosed. Finally we will perform a live demonstration with a blindly selected fetal model with heterotaxy
