Assessment of reproducibility and repeatability of cerebro-placental ratio.

INTRODUCTION

To test the reproducibility and repeatability of the cerebro-placental ratio.

STUDY DESIGN

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 fetal end by one examiner in a state of fetal quiescence, generating four measurements per fetus. Fetal middle cerebral artery PI was also obtained. Cerebro-placental ratio (CPR) was calculated as MCA PI/Umbilical artery PI. Variability of the CPR on the two sampling occasions was tested using Pitman test of equality of variance for related samples. The difference between the two sets of CPR measurements was plotted against the mean to generate 95% limits of agreement.

RESULTS

A total of 158 women were recruited. The mean CPR was significantly lower when the umbilical artery PI was obtained at the para-vesical site, than when it in obtained from a free loop (p < 0.001). No significant correlation was seen between gestational age and CPR, when the umbilical artery PI was measured from the para-vesical site (r = -0.079, p = 0.323) or the free loop (r = -0.103, p = 0.198). Total variance of the CPR using the umbilical artery free loop was 0.286, and that using the para-vesical site of the umbilical artery was 0.164. Pitman's test showed that the total variability of CPR at the two sites was significantly different (r = 0.254, p < 0.001). The variability of CPR was significantly lower if the umbilical artery PI measurement was taken at the fetal end than that in the free loop.

CONCLUSION

The mean CPR site was significantly lower when the umbilical artery PI was obtained at the para-vesical than in the free loop. Measurement site for the umbilical artery PI contributes to a significant proportion to the total variability of the cerebro-placental ratio. CPR measurements should include umbilical artery PI measurements at the para-vesical site rather than the free loop of the umbilical cord in order to improve repeatability. Appropriate reference ranges for the interpretation of CPR will be needed.