Is there a relationship between estimated fetal weight and umbilical artery Doppler impedance indices?

To establish whether there is a relationship between estimated fetal weight (EFW) and umbilical artery Doppler waveform impedance indices in the third trimester.

The pulsatility index (PI) and S/D (systolic/diastolic) ratio were obtained together with the EFW from 274 low-risk pregnancies. Measurements were made at fortnightly intervals from 30 weeks' gestation until delivery. A relationship between the two impedance indices and EFW was sought at gestational age ranges of 30-32, 33-35, 36-38 and 39-41 weeks.

There were 918 pairs of PI (with S/D) and EFW available for analysis. The mean of the impedance indices decreased with advancing gestational age as expected. There was no clinically relevant correlation between impedance indices and EFW within any of the gestational age ranges.

Impedance indices from the umbilical artery Doppler waveform decrease with advancing gestational age due, at least partially, to expansion of the placental vascular tree. Within narrow gestational age ranges in the third trimester, there is no meaningful correlation between fetal weight and impedance indices. It is therefore not necessary to adjust umbilical artery Doppler impedance indices to account for fetal size.