The effect of sampling site on the variability of Umbilical artery PI

To test the hypothesis that standardising the site of sampling of umbilical artery Doppler reduces the variability of umbilical artery Pulsatility Index (PI).

In this prospective study, pregnant women with a singleton pregnancy and secure dating were invited to participate after 24 weeks of pregnancy. Using recommended technique, umbilical artery PI was measured from the free loop of the umbilical cord and from the para-vesical site by the same examiner at the beginning and the end of ultrasound examination in a state of fetal quiescence, generating four measurements per fetus. Variability of the measurements at the two sampling sites was tested using Pitman test of equality of variance for related samples. The difference between the two sets of measurements were plotted against the mean to generate limits of agreement.

A total of 158 women were recruited. Umbilical artery PI was significantly negatively correlated with the gestational age (r = -0.246 for free loop and -0.262 for para-vesical site, both p < 0.005). The PI at the para-vesical site was significantly higher than in the free loop (p < 0.001). Pitman's test showed that the total variability of umbilical artery PI at the two sites was no different (r = -0.091, p = 0.254).

Measurement site contributes to an insignificant proportion to the total variability of the umbilical artery PI measurements. Umbilical artery PI is significantly lower when measured in the free loop as compared to the para-vesical site. Standardising the site of sampling does not improve the repeatability of umbilical artery PI measurement.