Supplementary Material for: Uterine Artery Pulsatility Index Assessment at 11-13<sup>+6</sup> Weeks' Gestation

<b><i>Introduction:</i></b> First-trimester uterine artery pulsatility index (PI) measurements form part of an algorithm used to assess the risk of developing pre-eclampsia. The objective of this study was to construct a population-specific reference range for both the lower and mean maternal uterine artery PI at 11-13<sup>+6</sup> weeks' gestation and to assess measurement agreement. <b><i>Materials and Methods:</i></b> Reference ranges for mean and lower PI measurements were developed using polynomial regression models following prospective collection of maternal uterine artery PI measurements at 11-13<sup>+6</sup> weeks' gestation. Measurement agreement studies were performed by two experienced operators. <b><i>Results:</i></b> Measurements from 298 women were included in the primary study. Polynomial regression indicated no change over gestational age for the lower PI (mean 1.44). There was an inverse relationship between the average PI and gestational age (mean [0.8960 + (2.9771 × CRL<sup>-1/2</sup>)]<sup>2</sup>). PI measurement agreement was good-strong (intraclass correlation (ICC) 0.50-0.79) between operators, and within-operator agreement was almost perfect (ICC 0.88-0.93). <b><i>Conclusions:</i></b> Reference ranges for both the average and lowest PI of the maternal uterine arteries were derived at 11-13<sup>+6</sup> weeks' gestation. This will provide a basis for development of auditable standards for first-trimester uterine artery Doppler measurements. The PI measurements are reproducible and reliable.