Doppler analysis of the umbilical artery: why we must prefer the free loop of the cord.

Aim: The main objective was to confirm with modern tools the former assertion that there is a vascular resistance gradient along the umbilical cord.

Methods: We performed a prospective observational study in a population of women with third trimester monofetal and uncomplicated pregnancies during two months. Pulsatility Index and Resistance Index were calculated on three sites: placental end, free loop and fetal end of the cord.

Results: Sixty-five pregnant women were included in the study. Mean gestational age was 32 weeks and 3 days. We confirmed the presence of a resistance gradient with a significant increase of both Resistance and Pulsatility Indices from the placental toward the fetal end of the cord (Resistance Index = 0.61±0.07 at placental end and 0.66±0.11 at fetal end [P<0.001]. Pulsatility Index = 0.97±0.17 at placental end, and 1.06±0.25 at fetal end [P<0.001]).

Conclusion: Resistances seem higher when doppler waveform analysis of the umibilical artery are performed at the fetal end of the cord. We recommend the free loop insonation site to obtain a better sensitivity. International guidelines strongly need to be well diffused in order to standardize practices.