Anatomical configuration of the spinal column in the supine position. II. Comparison of pregnant and non-pregnant women.

To assess the changes in the curvature of the spinal column in the supine position during pregnancy, we studied seven pregnant (32-37 weeks of pregnancy) and seven non-pregnant women using magnetic resonance imaging. T1-weighted sagittal midline magnetic resonance images of the spinal column were obtained with subjects in the supine position with left tilt. There was no significant difference in the maximum angle of decline of the lumbar spinal canal between the pregnant (mean 12.4 (SD 3.3) degrees) and non-pregnant (13.4 (3.9) degrees) groups. The maximum angle of incline of the upper thoracic spinal canal was smaller in the pregnant (15.8 (2.9) degrees) than in the non-pregnant (22.7 (6.0) degrees) group. The highest point of the lumbar spinal canal was located at a lower lumbar region in the pregnant (median L4-5 (range L4 to L4-5)) than in the non-pregnant (L4 (L3-4 to L4)) group. The lowest point of the thoracic spinal canal was located at a higher thoracic region in the pregnant (T6-7 (T6 to T7-8)) than in the non-pregnant (T8 (T6-7 to T9)) group. This study revealed that the apex of lumbar lordosis was caudad and thoracic kyphosis was reduced in the supine position in the later stages of pregnancy. These changes in the curvature of the spinal column may explain, in part, the enhanced cephalad spread of subarachnoid hyperbaric anaesthetic solutions in the later stages of pregnancy.