To explore the role of Oblique View eXtended (OVIX) method in three-dimensional (3D) ultrasound in positioning the fetal conus medullaris terminal.

In this study, 122 normal fetuses and 5 fetuses with spinal bifida at 20^th to 30^th gestation week from October 2014 to May 2015 in our consultation were included after a system ultrasound examination. The conus medullaris terminal of 3D volume data was collected. OVIX was used to form the coronal section of spine which contained conus medullaris terminal and at least above 5 vertebral bodies and position conus medullaris terminal. The development of nervous system of the normal fetuses was followed up till postnatal 28 days. According to the pathological diagnosis of spinal bifida cases after induced labor, the position of conus medullaris terminal in normal cases and spinal bifida cases was compared.

The fetal conus medullaris terminal could be located in 120 cases (94.5%, 120/127), while the conus medullaris terminals of 7 normal cases were failed to be located, including due to pregnant obesity in 3 cases and due to fetus position in 4 cases. The conus medullaris terminal was positioned in L₃ or above L₃ in 115 normal fetuses, below L₄ in 5 abnormal fetuses including in L₅ in 2 cases, in S₁ 2 in cases, and in S₃ 1 in case. OVIX results were consistent with the pathological diagnosis in the 5 abnormal cases.

OVIX could simultaneously display the osseous marks including 12^th rib, T₁₂ and the conus medullaris terminal. The fetal conus medullaris terminal can be exactly positioned by OVIX method, which may provide valuable imaging information for prenatal evaluation of spinal bifida.