·         Fetal respiration- Fetal respiration affects the maximum systolic and diastolic values and also affects the length of the cardiac cycle. Measurements should be taken with the fetus in a quiet non-breathing state.

·         Tachycardia-heart rates over 180/min exaggerate the diastolic component of the indices by not allowing ant 'run off' time.

·         Bradycardia-prolonged diastole allows excessive 'run off' time and thus diastole (max.) is artificially low.

·         Blood hematocrit- decreased fetal blood hematocrit in conditions such as Rh sensitization is associated with increased fetal blood velocity.

·         Blood viscosity- the umbilical artery systolic; diastolic ratio increases with increasing blood velocity at high shear rates but not at low rates. Similarly a significant negative correlation exists between umbilical venous blood flow and blood viscosity.                 

NB- correction methods taking heart rates into account are available. There is no significant difference in the waveforms obtained by continuous wave and pulsed wave Doppler.