Introduction
The left lateral-tilt position has become an established practice of obstetric anesthesia since the 1970s, especially after the study by Crawford et al.,1 who reported the significant improvement in the fetal acid-base status when a cushion was placed under the hip of mother to tilt the pelvis to the right or to the left under general anesthesia. We previously reported that the left and right lateral-tilt position at 15°did not reduce compression of the IVC, and the left lateral-tilt position at 30°, but not right lateral-tilt position at 30 °,  was effectively reducing compression of the IVC.  
Here, we used magnetic resonance (MR) imaging to examine the effect of the right lateral-tilt position on the IVC compression.  

Methods
MR images of 20 singleton parturients (37 weeks gestation) were obtained for observation of the inferior vena cava and aorta in both the supine, left lateral-tilt positions (15°, 30°) and right lateral-tilt positions (15°, 30°) with head to toe placement of a 1.8-m long hard V-block constructed of closed-cell polyethylene foam under the right side of the parturient's body (Fig.1). 

Results
In parturients, the abdominal aorta could be identified in any positions. Aortic volume did not change in any of positions. (Figs.2,3).
MR images revealed total compression of the IVC when parturients were in the supine positions (Figs.2,3). The IVC volume in the left lateral-tilt position was not increased at 15° (5±5 vs. 6±6 ml), whereas the corresponding values at 30° were significantly increased (13±10 ml), compared with the supine position (P<0.05). In contrast, the IVC volume in the right lateral-tilt position was increased not at 15° (7±7 ml) or at 30°(6±2 ml) (Fig.4). 

Conclusion
In parturients, the abdominal aorta is not compressed by pregnant uterus in any position.
Only left lateral-tilt position at 30° effectively reduces compression of the IVC .

The effect of right versus left lateral-tilt position on compression of the inferior vena cava in parturients