Reference ranges of uterine arteries pulsatility index from first to third trimester based on serial Doppler measurements: longitudinal cohort study

Objective: To provide gestational age (GA)-specific reference ranges for uterine artery (UtA) mean pulsatility index (PI) based on longitudinal data assessment throughout pregnancy. Methods: This was a prospective longitudinal cohort study of singleton low-risk pregnancies with adequate health and nutritional status at the time of enrollment and without any fetal anomaly, receiving prenatal care from January 2018 to July 2021 at the Maternal Fetal Medicine Unit of IRCSS San Raffaele Scientific Institute, Milan. Women were recruited at ≤ 12+6 weeks' gestation and then underwent serial standardized ultrasound monitoring including UtA-PI measurements, by experienced certified operators until delivery. A modulus exponential normal (MEN) model smoothed with fractional polynomial was implemented to model UtA-PI as a function of GA. Equations for mean (μ) and standard deviation (SD) of the estimated curves were calculated as well as GA-specific reference charts of centiles for UtA-PI from 10+0 to 39+0 gestational weeks. Results: 476 healthy, low-risk pregnant women were included. A total of 2045 ultrasound scans were available for analysis. For all the included women at least 3 measures were recorded (observations per woman: median 4, range 3-9). At the three trimesters, the mean±SD UtA-PI values were equal to 1.84 +/-0.55, 1.07 +/-0.38 and 0.78+/- 0.23 respectively. Goodness of fit assessment revealed that second-degree smoothing was the most accurate fractional polynomial for describing UtA-PI course throughout gestation; therefore, it was modelled in a multilevel framework for the construction of UtA-PI curves. There was a rapid and significant decrease in the mean UtA-PI before 16 weeks, then the slope decreased with smoother decrement and more stable values from 20 up until 39 weeks. The 3rd , 5th , 10th , 25th , 50th , 75th , 90th , 95th and 97th centiles according to GA for UtA-PI are provided, as well as equations to allow calculation of any value as a centile. Conclusions: UtA-PI shows a progressive non-linear decrease throughout pregnancy. The new reference ranges for GA-specific UtA-PI constructed with rigorous methodology may have a better impact than previous models for screening placenta-associated diseases in the early stages of pregnancy and yet also for evaluating patients at later potential risk for pregnancy-induced hypertension and/or small for gestational age fetuses. This article is protected by copyright. All rights reserved.