BACKGROUND: Combined spinal-epidural anesthesia (CSEA) has gained an increasing interest as it combines the reliability of a spinal block and the flexibility of an epidural block. This study was designed to determine the distance from skin to epidural space (S-EP) and the distance from epidural space to subarachnoid space (E-SA) in obstetric parturient and whether weight, height, BMI (body mass index) and PI (ponderal index) might influence S-EP and E-SA. METHODS: Sixty obstetric patients undergoing elective cesarean section during CSEA in L3-4 level were partitioned into group I (with dural click), group II (no dural click) and in each group, weight (prepregnancy, pregnancy), height, BMI and PI were measured. Measurements of S-EP and E-SA were made. The Pearson correlation was used to investigate relationships between S-EP, E-SA and patient characteristics (height, weight, BMI, PI). RESULTS: S-EP was 4.39+/-0.49 cm and E-SA was 8.23+/-2.05 mm (7.99+/-2.07 mm in group I, 8.78+/-1.94 mm in group II). S-EP was related to weight, BMI and PI except height. E-SA was related to pregnancy BMI and pregnancy PI in group I. No correlation was found between E-SA and other variables in group II. CONCLUSIONS: E-SA in obstetric parturients was related to obesity and has been found to be somewhat larger and more variable due to the segmented and discontinous nature of epidural space.