We prospectively examined the distance from the skin to the epidural space (SE distance) in 95 Japanese parturient women who underwent epidural anesthesia at the L1-2 interspace, and studied the correlation between various physical factors and SE distance. The same anesthesiologist inserted the epidural tubing with the patient in the right lateral decubitus position. An epidural needle was introduced into the skin vertically via the midline approach. The epidural space was located using the loss-of-resistance technique. SE distance was measured to the nearest 0.5 cm using centimeter markings on the shaft of the epidural needle. The median value of SE distance was 3.5 cm, with a range of 2.5 to 6.5 cm, and in 800f cases SE distances were 3 to 4 cm. The correlation of body weight with SE distance was the highest of the physical factors (r^2 = 0.800, p = 0.0001), and a simple regression equation was formulated to aid in predicting SE distance: 'SE distance (cm) = 0.05 × body weight (kg) + 0.36'. This formula will be a useful clinical guide for administering epidural anesthesia in Japanese parturient women. In conclusion, the SE distance in most Japanese parturient women is between 3 and 4 cm at the L1-2 interspace and this value is most closely correlated with their body weight
