During normal pregnancy, physiological adaptations occur in the mother which assure an adequate blood supply to the fetus. Vascular resistance, mean arterial pressure, and sensitivity to endogenous constrictors are reduced, and cardiac output, heart rate, and blood volume are increased. Alteration of these haemodynamic adaptations to pregnancy is associated with conditions such as pre-eclampsia and intrauterine-growth retardation. 1 The mechanism responsible for maintaining low vascular resistance are still unknown, although several locally produced vasoactive agents are suggested to be implicated in the regulation of fetoplacental circulation.