Dopplerometry in Obstetrics: PI of Uterine Arteries, Umbilical Artery, MCA, and Cerebroplacental Ratio
Key Doppler Indices
The main targets of Doppler assessment in pregnancy monitoring are the umbilical artery (UA), uterine arteries (UtA), and the fetal middle cerebral artery (MCA). Indices are calculated as pulsatility index (PI), resistance index (RI), and systolic-diastolic ratio (S/D).
Reference Ranges for PI
Gestation-specific reference ranges for the PI of the umbilical artery and middle cerebral artery, as well as the cerebroplacental ratio, are presented in the work of the Fetal Medicine Foundation (Ciobanu A, Wright A, Syngelaki A, Wright D, Akolekar R, Nicolaides KH. Ultrasound Obstet Gynecol. 2019;53:465-472). Reference ranges for the mean PI of uterine arteries at 11–41 weeks of gestation are published by Gomez O, Figueras F, Fernandez S, et al. (Ultrasound Obstet Gynecol. 2008;32(2):128-132). International gestation-specific centiles for Doppler indices of the umbilical artery are provided in the INTERGROWTH-21st project (Drukker L, Staines-Urias E, Villar J, et al. Am J Obstet Gynecol. 2020;222(6):602.e1-602.e15). Specific numerical threshold values are not provided in the given excerpts [clarify].
Cerebroplacental Ratio (CPR)
The cerebroplacental ratio (CPR) is calculated as the ratio of MCA-PI to UA-PI. Reference ranges are presented by the Fetal Medicine Foundation (Ciobanu et al., 2019).
MCA-PSV and Fetal Anemia
According to current SMFM recommendations, the peak systolic velocity in the middle cerebral artery (MCA-PSV) is the primary non-invasive method for detecting fetal anemia and has replaced the previous reliance on determining bilirubin in amniotic fluid during routine monitoring. MCA-PSV values are often expressed in multiples of the median (MoM).
Features in Twins
In monochorionic twins, the intertwin difference in MCA-PSV (delta MCA-PSV) is used to predict twin anemia-polycythemia syndrome (TAPS) — a new antenatal classification (Tollenaar LSA, et al. Ultrasound Obstet Gynecol. 2019;53(6):788-793) [ISUOG Practice Guidelines: twin pregnancy, 2025].
Influence of Physiological Factors on MCA-PI
It should be considered that MCA-PI changes with fetal activity: the decrease in MCA-PI during fetal activity is primarily due to an increase in end-diastolic velocity (MCA-EDV) and to a lesser extent due to a decrease in MCA-PSV. A decrease in MCA-PI is noted for every 10 beats increase in fetal heart rate.
Frequently asked questions
How is the cerebroplacental ratio calculated?
CPR = MCA-PI / UA-PI. Reference ranges are published by the Fetal Medicine Foundation (Ciobanu et al., 2019).
What is the primary method for detecting fetal anemia?
According to SMFM recommendations, the assessment of MCA-PSV is primary; it has replaced the determination of bilirubin in amniotic fluid during routine monitoring.
Where can reference values for the PI of uterine arteries be found?
Reference ranges for the mean PI of uterine arteries at 11–41 weeks are provided by Gomez O, et al. (Ultrasound Obstet Gynecol. 2008;32(2):128-132).
How is MCA-PSV used in twins?
In monochorionic twins, the intertwin difference in MCA-PSV (delta MCA-PSV) is used to predict twin anemia-polycythemia syndrome (TAPS) [ISUOG, 2025].
Does fetal activity affect MCA-PI?
Yes, MCA-PI decreases with fetal activity primarily due to an increase in MCA-EDV; PI also decreases with an increase in fetal heart rate.