To provide a gestation age- and weight-specific mathematical formula for predicting the optimal depth of spinal needle insertion.

The study included 127 neonates between 28 and 40 weeks of gestation and weighing 700 to 4000 grams, and a total of 202 ultrasound examinations were performed. Anterior and posterior borders were delineated using ultrasound and measured as spinal canal depth in lateral decubitus position at L3–L4 vertebral interspace. The mid-spinal canal depth (MSCD) was calculated.

Spinal canal dimensions showed an increasing trend with an increase in weight and post-menstrual age of the babies. The best correlation was found between weight and MSCD with an r2 of 0.85, which is given by the formula MSCD (cm) = 0.2 × weight in kg + 0.45.

Knowledge of the spinal canal depth using the formula may facilitate accurate needle placement, thereby decreasing traumatic lumbar puncture.