Investigating the Novel Observation of Right-left Difference in Uterine Artery Vascular Resistance in Pre-eclampsia

The investigators previously demonstrated that epidural ropivacaine reduces uterine artery vascular resistance in a dose dependent manner in pre-eclampsia. The investigators also noted a marked right-left difference in vascular resistance between paired uterine arteries, which was almost completely abolished following epidural ropivacaine. However, this novel observation was not a stated outcome variable before the study began. This study assesses right-left difference in vascular resistance between paired uterine arteries as a primary end-point, assesses the dose-response effect of epidural lidocaine and compares the effect observed in pre-eclampsia with that in two control populations (term normal pregnancy and non-pregnant controls).