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 (R  = .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.