Abstract
OBJECTIVE: To establish a comprehensive reference range for first-trimester placental three-dimensional (3D) single-vessel fractional moving blood volume (svFMBV) using the OxNNet toolkit, based on values observed in healthy pregnancies. METHODS: This study utilized data from the First-trimester Placental Ultrasound (FirstPLUS) study, a longitudinal observational cohort study conducted between March and November 2022 at King's College Hospital, London, UK. Participants underwent 3D ultrasound assessment of the placenta, including power Doppler imaging, during routine first-trimester screening. The OxNNet toolkit was used for automated placental segmentation and 3D-svFMBV calculation. Quality control was performed in three stages to ensure image completeness and segmentation accuracy. Quantile regression and lambda-mu-sigma (LMS) modeling were used to construct reference charts for 3D-svFMBV. Model fit was assessed using the Akaike information criterion, and centile curves were constructed. RESULTS: The final cohort comprised 2547 cases. Visual assessment of histograms and quantile-quantile plots revealed positive skew in 3D-svFMBV determined at five specific locations within the uteroplacental vasculature. LMS modeling provided the best fit for constructing centile charts, with the Box-Cox Power Exponential original distribution used in most cases. The resulting centile charts demonstrated close agreement between predicted and observed centiles, with minimal deviation across all target centiles. CONCLUSIONS: This study provides novel reference ranges for first-trimester placental 3D-svFMBV at five locations within the uteroplacental vasculature. These findings offer a valuable foundation for future research into placental function and pregnancy outcome. © 2026 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.