Fetal umbilical artery flow velocity waveforms and placental resistance: pathological correlation

Placental microvascular anatomy was correlated with antenatal assessment of the umbilical circulation in 106 patients to further validate the measurement of the A/B ratio (the ratio of peak systolic to least diastolic flow velocity) of the umbilical artery flow velocity time wave‐forms as an index of blood flow resistance. Three groups of patients were studied: a normal group of 38 uncomplicated pregnancies, a control group of 33 potentially ‘at risk’ pregnancies with a normal A/B ratio matched by risk factors and gestation with the third group of 35 pregnancies with a high A/B ratio. Placental arterial resistance was quantitated by counting the number of small muscular arteries (