The placental resistance index: (S-D)/S (S: represents the amplitude of the systolic peak and D: the amplitude of the telediastolic peak) is constantly inferior to one from the 14th-16th week of amenorrhea during a normal pregnancy. The diastolic umbilical arterial blood flow is continuous because of low placental vascular resistance. From a clinical study of 21 patients with an absent or reversed end diastolic flow in umbilical artery waveforms, an obliterative process in the placental vascular tree was observed. A computer model was used to stimulate the reversed end diastolic flow in umbilical artery waveforms when the placental resistance increased. This haemodynamic approach shows that the placental blood flow decreased, and that the arterial blood pressure, as well as the pulse wave velocity increased when the placental resistance increased. The increase in the difference between the umbilical resistance and the placental resistance caused a reduction of the diastolic flow owing to a progressive increase of the pulse wave velocity reflected. It was concluded that there is a relation between perturbations in placental vascular bed and the flow in umbilical artery waveforms.