To investigate the application value of right-hand rule in determining left-right axis and fetal situs in prenatal MRI.

The prenatal MRI data of 254 fetuses were included retrospectively in West China Second University Hospital, Sichuan University from December 2017 to February 2021, which were followed by clear postpartum diagnosis. Fetal left-right axis and situs (orientation of gastric vacuole, liver and heart) were determined by the same radiologist blindly using general experience and right-hand rule respectively, meanwhile, results and time of each case were recorded. The right-hand rule was described in detail below: use your right hand to make a fist, and with thumb outstretched; the fetal head was represented by your right fist, the forearm represented the fetal body, the dorsal side of your fist represented the fetal occiput and spine column, the palm side represented the fetal belly, and your four fingers represented the fetal face, then the left side of fetus was exactly the direction your thumb pointing to. The postpartum imaging was used as the gold standard for situs anomalies. The time to determine the fetal left-right axis was compared using paired t test for statistical analysis.

Four fetuses were found situs anomalies in postpartum imaging. There were 1 missed case and 1 misdiagnosed case when using general experience to determine fetal situs, neither missed case nor misdiagnosis were present when using right-hand rule. The average time of using general experience or right-hand rule to determine the fetal left-right axis and situs were (35±6)s and (24±9)s respectively, and the difference was statistically significant (t=20.65, P<0.001).

The right-hand rule is an accurate and efficient method for determining left-right axis and situs in prenatal MRI, which can effectively reduce missed diagnosis and misdiagnosis of situs anomalies, especially in situs inversus totalis.

Fetus; Magnetic resonance imaging; Right-hand rule; Left-right axis; Situs anomalies