Longitudinal reference ranges for Doppler blood velocity parameters of inferior adrenal artery in the human fetus

ABSTRACT Objective Blood flow to the fetal adrenal arteries is shown to increase during placental dysfunction in animal studies. Reference ranges of fetal adrenal blood flow parameters are needed to explore the diagnostic possibilities of this concept in humans. The aim of this study was to establish longitudinal reference ranges for pulsatility index (PI), peak systolic velocity (PSV), time‐averaged maximum velocity (TAmax) and end‐diastolic velocity (EDV) of the inferior adrenal artery of human fetuses across the second half of gestation. Methods This was a prospective longitudinal cohort study conducted at Stavanger University Hospital, Stavanger, Norway, of women with a singleton low‐risk pregnancy presenting for routine second‐trimester screening between June 2020 and May 2023. Participants were examined every 4 weeks, from 19 to 41 weeks' gestation, to establish longitudinal reference ranges for the abovementioned blood velocity parameters, covering the second half of pregnancy. We used the visualization and blood flow velocity measurement techniques, as well as the definition of the inferior adrenal artery, as described in our previous study. Each assessment included ultrasound examination with SlowFlow™ power Doppler to visualize the fetal inferior adrenal artery and pulsed‐wave Doppler to record blood velocity parameters in this artery, with one to six assessments per participant over the course of the second half of pregnancy. At each assessment, PSV, EDV and TAmax were measured and the PI was calculated. Mixed linear models were used to estimate the longitudinal reference ranges. Results In total, 146 participants were examined and the blood velocity in the fetal inferior adrenal artery was measured successfully in 542/608 examinations, yielding a success rate of 89%. Based on this dataset, we constructed longitudinal reference ranges for PI and PSV, as well as for EDV and TAmax, of the fetal inferior adrenal artery throughout the second half of gestation. The PI of the fetal inferior adrenal artery increased slightly throughout the second trimester and declined towards term. The PSV of the fetal inferior adrenal artery was stable throughout the second trimester, but increased from 30 weeks' gestation until the end of the third trimester. Conclusion We present longitudinal reference ranges for PI and PSV of the fetal inferior adrenal artery in low‐risk singleton pregnancies across the second half of pregnancy. Additionally, we provide terms, including formulae and the conditions under which they apply, for calculating individualized conditional reference ranges for serial measurements of blood velocity parameters. © 2026 International Society of Ultrasound in Obstetrics and Gynecology.