Monochorionic diamniotic twins use the Klaritsch (2009) reference formula.
Enter gestational age in complete weeks. Singleton range: 14-40 weeks. MCDA twins: 18-36 weeks.
Days 0-6 beyond the completed weeks, for sub-week precision.
Enter the highest peak systolic velocity from at least 3 measurements, in cm/s.
Post-IUT threshold is 1.69 MoM instead of 1.50 MoM, due to donor erythrocyte effect on MCA velocity.
Enter MCA-PSV Values
Select gestational age and enter the measured MCA-PSV to calculate the MoM value and fetal anemia risk classification.
How to Use the MCA-PSV Calculator
Select Gestational Age
Choose the gestational age in complete weeks using the dropdown (range: 14–40 weeks for singletons, 18–36 weeks for MCDA twins). For sub-week precision, also select the number of additional days (0–6). The calculator uses your exact gestational age to compute the expected median MCA-PSV from the Mari or Klaritsch formula.
Enter the Measured MCA-PSV
Type the highest peak systolic velocity in cm/s, recorded from at least 3 MCA Doppler measurements. Ensure the measurement was taken at 0° insonation angle, with the sample volume placed 2 mm from the ICA origin. Avoid measurements taken during fetal breathing or significant fetal activity, as these can falsely elevate the reading.
Select Pregnancy Type and Post-IUT Status
Toggle between Singleton (Mari formula) and MCDA Twins (Klaritsch formula). If the patient has recently received an intrauterine transfusion, enable the Post-IUT toggle — this adjusts the action threshold from 1.50 to 1.69 MoM, reflecting altered MCA-PSV physiology after donor erythrocyte transfusion.
Review MoM, Risk Classification, and Serial Trend
The calculator displays the MoM value, expected median, anemia risk classification, and a visual gauge showing your result relative to the 1.29, 1.50, and 1.55 MoM thresholds. Use the serial tracker to log multiple measurements over time and visualize the trend toward or away from the critical 1.50 MoM threshold. Export results as CSV for documentation or print for clinical records.
Frequently Asked Questions
What is the clinical significance of the 1.50 MoM threshold?
A MCA-PSV at or above 1.50 MoM is the primary clinical action threshold for fetal anemia. The landmark Mari et al. study (NEJM 2000) demonstrated that this cutoff detects moderate-to-severe fetal anemia — defined as hemoglobin more than two standard deviations below the mean for gestational age — with 100% sensitivity. The false-positive rate is 12%, meaning 1 in 8 fetuses classified as anemic at this threshold will not have true anemia on cordocentesis. This high sensitivity with acceptable false-positive rate makes it the standard of care for non-invasive fetal anemia surveillance, avoiding unnecessary invasive procedures in most cases while ensuring timely intervention for all genuinely anemic fetuses.
Why does the threshold change to 1.69 MoM after intrauterine transfusion?
After an intrauterine transfusion (IUT), the fetal circulation contains a mixture of the fetus's own erythrocytes (which may be anemic) and donor erythrocytes (with normal hemoglobin). This mixture has different viscosity and oxygen-carrying properties than pure fetal blood, altering the relationship between MCA-PSV and fetal hemoglobin. Studies have shown that after IUT, the same degree of true anemia produces a higher MCA-PSV than in non-transfused fetuses. Using the standard 1.50 MoM threshold post-IUT would lead to excessive false-positive diagnoses of recurrent anemia. The 1.69 MoM threshold has been validated for post-transfusion monitoring and should be applied until several weeks after the last transfusion.
How does this calculator differ for MCDA twin pregnancies?
In monochorionic diamniotic (MCDA) twin pregnancies, the baseline MCA-PSV is lower than in singleton pregnancies at the same gestational age, likely due to differences in hemodynamic adaptation in twins. Using the singleton Mari formula for MCDA twins would systematically overestimate the MoM, leading to false-positive anemia diagnoses. This calculator uses the Klaritsch et al. (UOG 2009) formula for MCDA twins: Median = e^(2.17 + 0.049 × GA), which was derived from a reference population of uncomplicated MCDA twin pregnancies. MCDA twins are particularly at risk for fetal anemia from twin-to-twin transfusion syndrome (TTTS) and twin anemia-polycythemia sequence (TAPS), making accurate MCA-PSV reference values especially important in this population.
What causes false-positive MCA-PSV results?
False-positive MCA-PSV readings (elevated MoM without true anemia) can occur in several situations. Fetal breathing movements and significant fetal activity transiently increase MCA blood flow velocity and should be waited out before measurement. Maternal meals within 1-2 hours of scanning can also increase fetal cerebral blood flow. Technical errors such as non-zero insonation angle, incorrect sample volume placement, or not selecting the highest of three measurements can give inconsistent results. After 35 weeks' gestation, the false-positive rate increases substantially, so MCA-PSV is less reliable in late pregnancy. Borderline results (MoM between 1.45 and 1.55) should be repeated within 24-48 hours before clinical decisions are made.
How often should MCA-PSV be measured in a high-risk pregnancy?
The monitoring frequency depends on the clinical indication and the most recent MoM value. For red blood cell alloimmunization, the AlloHope Foundation and SMFM guidance recommend initiating MCA-PSV surveillance at 16-18 weeks for high-risk cases (titer ≥ 1:16 for anti-D or any level for anti-Kell). If MoM is below 1.29, weekly measurements are standard. If MoM is approaching 1.4-1.49, reassessment every 2-3 days is appropriate. If MoM reaches or exceeds 1.50, immediate referral for cordocentesis and potential intrauterine transfusion is indicated. For TTTS and TAPS monitoring, frequency is guided by the stage of the syndrome and the treating MFM specialist's clinical judgment.
Can MCA-PSV be used before 18 weeks or after 35 weeks?
The Mari formula was validated primarily for gestational ages 18-40 weeks, and most clinical use is within this range. Some references, including Perinatology.com, extend the singleton range to 14 weeks. Our calculator supports 14-40 weeks for singletons following this extended range. However, clinical use below 18 weeks is less validated, and the clinical threshold of 1.50 MoM should be interpreted with caution. After 35 weeks, the false-positive rate increases significantly — the velocity increases more steeply with gestational age in late pregnancy than the Mari formula predicts, so apparent elevations in MoM are more likely to be false positives. Clinicians typically weigh the MCA-PSV result alongside amniotic fluid assessment and clinical context when interpreting results at advanced gestational ages.