Effects of the pregnant uterus on the extradural venous plexus in the supine and lateral positions, as determined by magnetic resonance imaging

To clarify the effects of the pregnant uterus on the extradural venousplexus in the supine and lateral positions, we studied magnetic resonance(MR) images of the lumbar spine in three parturients. T2- weighted axial MRimages were obtained with the parturient in the supine and lateralpositions. On each slice level in the same subject, the MR images werecompared with the control MR images obtained before pregnancy. When theparturient lay supine, the pregnant uterus compressed the inferior venacava and almost totally obstructed it; the extradural venous plexus wasengorged. On turning the parturient into the lateral position, the inferiorvena cava was free from compression, and the engorged extradural venousplexus was found to shrink to the level of the non-pregnant state.