Normal ascent of the conus medullaris: a post-mortem foetal MRI study

The position of the conus medullaris is considered abnormal if it ends below lumbar vertebrae three (L3) at birth. We used magnetic resonance imaging (MRI) to measure the position of the conus in post-mortem foetuses, to investigate the timing of normal ascent.

The position of the conus in 84 post-mortem foetuses (mean gestation 26.3 weeks; range 14-41 weeks) was identified using 3D MRI datasets. A numerical scale was used for vertebral levels, from 1 (S2) to 15 (T12).

There was significant ascent of the conus medullaris with increasing gestation. At 20 weeks gestation, an estimated 84.2% (95% confidence interval (CI): 72.9, 93.2%) of foetuses have a conus position of L4/5 or higher, but only 22.8% (95% CI 11.7, 34.9%) at L3 or higher. By 26 weeks, an estimated 50.7% (95% CI 34.1, 67.5) will have reached L3, and 94.8% (95% CI 87.0, 98.5%) reach L3 by 40 weeks.

There is regular ascent of the conus throughout foetal life. Although growth for each individual foetus may be non-linear, most foetuses have a conus level within the normal adult range by 33 weeks gestation.