Background The intercristal line, defined by the superior aspect of the iliac crest, is used to clinically identify the correct position for lumbar puncture in neonates. Accepted practice is to insert the needle in the intervertebral space at L3/4 or L4/5.

Aim We hypothesised variation in anatomical structures in term neonates and that the intercristal line might not accurately aid identification of the correct intervertebral space.

Method Following ethical approval 30 term neonates were recruited. Paediatricians identified and marked the intercristal line and intervertebral space above, with neonates in left lateral position. The anatomical position of both points and the conus medullaris end point were confirmed using ultrasonography.

Results The intercristal line was marked from L2/3 to L5/S1. In 25 neonates (83%) the intercristal line was identified between L3/4 and L4/5. The intervertebral space above this line was marked between L1/2 to L4/5. The potential site for lumbar puncture was identified too high in 11 cases (36%).

The end of the conus medullaris ranged from L1 to L3 terminating at L2 or lower in 11 cases (36%).

Conclusion There is wide variation in the position of the intercristal line and potential lumbar puncture site. The potential lumbar puncture site was marked higher than anticipated at L2/3 or above in 11 neonates (36%). The end of the conus medullaris was identified in 11 neonates (36%) at L2 or lower. Using the intercristal line to guide lumbar puncture does not appear to be accurate raising the possibility of potential spinal cord damage.