Sonographic evaluation of the fetal conus medullaris at 20 to 24 weeks' gestation.

Objective: To determine the normal level of the conus medullaris (CM) of the spinal cord of normal fetuses at 20 to 24 weeks' gestation using abdominal sonography.

Methods: Using a 4 to 8 MHz ultrasound abdominal transducer, CM was imaged in the midsagittal plane with the fetal spine facing toward the transducer. The CM ending level was located by counting the vertebrae caudal starting at T12, which was identified by lowest fetal rib. The CM locations were defined according to their positions relative to the vertebrae.

Results: One hundred and ten consecutive pregnant women were included in the study. The CM was clearly demonstrated in 78 (71%) of these cases. Most of the scanned fetuses had the CM ending height adjacent to vertebrae L2, L2-3 inter-space and L3 (73/78, 93%). The L1-2, L2, L2-3, and L3 vertebral groups did not differ in mean gestational weeks, maternal age, or fetal gender.

Conclusion: The level of the CM can be determined at 20 to 24 weeks' gestation in about 70% of fetuses. Most of the scanned fetuses had the CM ending height adjacent to vertebrae L2, L2-3 vertebral space, and L3 (73/78, 93%). When detected lower than the third lumbar vertebra it should prompt prenatal and postnatal surveillance.