 During a caesarean section the blood pressure is usually measured every few minutes and it may fall too quickly to be detected in a timely fashion by this intermittent means. The monitor the Masimo Rainbow SET MRS uses a peg attached to a finger to detect amongst other things the flow of blood through it. One of the measures it takes is called the Pleth Variability Index PVI which unlike standard blood pressure measurement it measures continuously. The investigators would like to try the MRS on patients to see if the PVI can be used to predict falls in blood pressure. 