Placental Assessment Using Microvascular Flow Imaging: Reference Ranges and Clinical Application

To establish gestational age-specific reference ranges for the intraplacental vascular index measured using microvascularity-flow ultrasound software (VI^MV) and to evaluate its variation by placental location, parity, and pregnancy outcome.

This prospective study enrolled 354 singleton pregnancies, of which 243 (68.64%) uncomplicated cases were used to construct reference ranges. A residual bootstrap method with 500 iterations was applied to square-root-transformed VI^MV values, followed by back-transformation to derive the 5th, 50th, and 95th percentiles with 95% confidence intervals. Group differences were assessed using quantile regression, and associations with Doppler parameters were examined.

VI^MV increased significantly with advancing gestational age (linear coefficient 1.88, p = .007), following a curvilinear pattern with mild late-gestation downturn (quadratic coefficient - 0.036, p = .011). Posterior placentas showed slightly higher early values, although this difference was not significant; from 31 weeks onward, anterior placentas demonstrated significantly higher VI^MV (mean difference + 15.4, p = .010 at 31-35 weeks; +11.6, p = .003 at 36-40 weeks). Multiparous women showed a modestly higher fitted trajectory and a similar non-significant trend toward higher birthweight, but parity was not independently associated with VI^MV. Pregnancies complicated by maternal disease, hypertensive disorders, or small-for-gestational-age neonates exhibited lower trajectories than uncomplicated pregnancies, although these differences were not statistically significant. VI^MV was inversely associated with uterine and umbilical artery pulsatility indices and positively associated with umbilical vein time-averaged mean velocity.

Gestational age-specific VI^MV reference ranges were established. VI^MV may provide an adjunctive quantitative marker for assessing placental vascular adaptation and identifying pregnancies with altered placental perfusion.