A method for quantitative 2-dimensional sonographic analysis of the fetal conus medullaris position.

OBJECTIVE: This study explored the use of 2-dimensional (2D) ultrasound scans for the quantitative assessment of the fetal conus medullaris (CM) position and its correlation with gestational age (GA).

METHODS: This was a prospective study. We identified the first sacral vertebra (S1) by intersection of 2 lines in 2D scans, then counted upward from S1 to determine the CM level and recorded the number of ossified sacral vertebral bodies. A quantitative assessment of the CM position was performed by measuring the distance between the CM and the midpoint of the S1 (CM-S1). The correlation between the CM-S1 distance and GA was evaluated.

RESULTS: We determined the CM level by identifying S1 first in 521 fetuses (GA, 20-38 weeks). The CM position in 70% of cases was at the L2 and L2-3 level, and at the L2 level or above after 37 weeks. The number of ossified sacral veterbral bodies was not consistent. CM-S1 measurements were easy to perform. A significant positive correlation between CM-S1 distance and GA was observed (R2  = .89, P < .05). The best-fit formula was: CM-S1 distance = 1.57 × GA - 16.43. The normal reference range was established and the fifth percentile was calculated for each GA.

CONCLUSIONS: S1 was easily identified, and the CM position relative to S1 was useful. There was a substantial correlation between CM-S1 and GA. Below the fifth percentile it was suggested that tethered cord may exist.