Sonographic evaluation of the fetal conus medullaris.

Objective: The aim of this study is to determine the most reproducible method in the sonographic evaluation of the conus medullaris (CM) and its relationship with gestational age (GA).

Methods: This is a prospective study of singleton structurally normal fetuses between 20 and 30 weeks' gestation. Sonographic evaluation of the CM was performed using two methodologies: a qualitative assessment of the CM level in relation to the lumbar vertebrae and a quantitative measurement of the distance from the CM to the last spine ossification centre (conus-sacrum or CS distance). Both parameters were analysed offline by two operators using 3D stored volumes. Interobserver variability of methods, the relationship between CS and femur length (FL) or GA was evaluated.

Results: We analysed 101 3D volumes. Interobserver concordance was low for the CM level (k = 0.4, P < 0.05) and high for CS distance (ICC = 0.950). A significant correlation between CS and both FL and GA was observed.

Conclusions: CS distance but not CM level is reproducible. CS distance is significantly correlated with both FL and GA. CS distance could be useful in the assessment of prenatal skin-covered spinal dysraphism.