RESUMO
Twenty nine term pregnant women were subjected to Doppler ultrasound study of the umbilical artery flow velocity waveform and the systolic diastolic ratio [S/D] was calculated. The pregnancy was uncomplicated in 15 women while 14 had preeclampsia. Women were classified into normal [<3] and abnormal >3 S/D ratio. After delivery, infants and placentas were weighed, number of small muscular arteries per tertiary stem villus was counted and cord length as well as the average diameter of the umbilical arteries were measured. In a multiple regression analysis, none of the above variables in addition to maternal age, had significant influence on the S/D ratio. When groups were compared, again none of the variables showed significant difference between the group with normal and the group with abnormal S/D ratio. The S/D ratio and cord length were comparable in the preeclamptic and normal women, while the number of arteries per villus was fewer in preeclamptic cases [P<0.05] who also had wider umbilical artery diameter [P< 0.05]. We conclude that the relation between placental vascular resistance and the umbilical artery S/D ratio is not a simple one as each may be altered independent from the other. This must be considered when measurement of the umbilical S/D ratio is clinically interpreted