MCA Calculator
Estimate your mcacalculator with our free gynecology & pregnancy calculator. See reference ranges, risk factors, and next-step guidance.
Formula
MoM = Measured MCA-PSV / Median MCA-PSV; Median = 10^(0.4437 + 0.0414 x GA)
The Multiples of the Median (MoM) is calculated by dividing the measured peak systolic velocity by the expected median for the gestational age. The median is derived from the Mari regression formula. MoM above 1.5 indicates moderate to severe fetal anemia with near 100% sensitivity.
Worked Examples
Example 1: Normal MCA-PSV at 28 Weeks
Problem: A fetus at 28 weeks 0 days has an MCA-PSV measurement of 40 cm/s. The mother has anti-D antibodies. Calculate MoM and assess anemia risk.
Solution: Gestational age: 28.0 weeks\nMedian MCA-PSV = 10^(0.4437 + 0.0414 x 28.0)\n= 10^(0.4437 + 1.1592)\n= 10^1.6029\n= 40.1 cm/s\nMoM = 40 / 40.1 = 1.00\n1.5 MoM threshold = 40.1 x 1.5 = 60.1 cm/s\n1.29 MoM threshold = 40.1 x 1.29 = 51.7 cm/s\nAssessment: Normal (MoM < 1.29)
Result: MoM: 1.00 | No Anemia Detected | Normal zone | Continue routine surveillance every 1-2 weeks
Example 2: Elevated MCA-PSV Suggesting Severe Anemia
Problem: A fetus at 32 weeks has an MCA-PSV of 75 cm/s in a pregnancy complicated by anti-Kell antibodies. Previous reading 2 weeks ago was 55 cm/s.
Solution: Gestational age: 32.0 weeks\nMedian MCA-PSV = 10^(0.4437 + 0.0414 x 32.0)\n= 10^(0.4437 + 1.3248)\n= 10^1.7685\n= 58.7 cm/s\nMoM = 75 / 58.7 = 1.28 -> rounded 1.28\nWait, recalculating: MoM = 75 / 58.7 = 1.28\n1.5 MoM threshold = 58.7 x 1.5 = 88.1 cm/s\nTrend: (75 - 55) / 55 x 100 = 36.4% increase\nAssessment: Borderline elevated, close to mild anemia threshold
Result: MoM: 1.28 | Borderline - approaching mild anemia threshold | 36.4% increase from prior | Recommend repeat in 1 week
Frequently Asked Questions
What is MCA Doppler and what does it measure?
Middle Cerebral Artery (MCA) Doppler ultrasonography measures the peak systolic velocity (PSV) of blood flow in the fetal middle cerebral artery, one of the major blood vessels supplying the fetal brain. This non-invasive test has become the gold standard for detecting fetal anemia without the risks associated with invasive procedures like cordocentesis. When a fetus becomes anemic, the blood becomes less viscous (thinner) and the heart pumps faster to compensate, resulting in increased blood flow velocity that is detectable on Doppler ultrasound. The measurement is taken with the ultrasound transducer positioned to visualize the Circle of Willis, sampling the MCA near its origin from the internal carotid artery at an angle as close to zero degrees as possible. The highest peak velocity is recorded and compared to established reference ranges for the specific gestational age.
How is MCA-PSV interpreted using Multiples of the Median (MoM)?
MCA-PSV values are converted to Multiples of the Median (MoM) to standardize interpretation across different gestational ages, since normal blood flow velocity increases as the fetus grows. The MoM is calculated by dividing the measured PSV by the expected median PSV for that gestational age, derived from reference tables established by Mari and colleagues. An MoM of 1.0 means the measured value equals the expected median. Values above 1.5 MoM have been established as the critical threshold for detecting moderate to severe fetal anemia, with a sensitivity of nearly 100 percent, meaning virtually all cases of significant anemia will be identified. Values between 1.29 and 1.5 MoM suggest possible mild anemia warranting closer surveillance. This threshold-based system allows clinicians to make consistent management decisions regardless of gestational age.
How often should MCA Doppler surveillance be performed?
The frequency of MCA Doppler surveillance depends on the clinical scenario and the degree of alloimmunization risk. In pregnancies with significant red cell antibodies (anti-D titer above 1:16 or anti-Kell at any detectable level), MCA monitoring typically begins at 16 to 18 weeks and is repeated every 1 to 2 weeks. The interval may be shortened to weekly or twice-weekly when MCA-PSV values are trending upward or approaching the 1.5 MoM threshold. For pregnancies with lower-risk antibody levels, monitoring may begin later and occur less frequently. After 35 weeks of gestation, MCA-PSV becomes less reliable for predicting anemia due to physiological changes in fetal circulation, and clinical decision-making may shift toward delivery planning. Serial measurements are more valuable than isolated readings, as the trend over time provides important information about the rate of fetal red cell destruction.
What happens when MCA-PSV exceeds the 1.5 MoM threshold?
When MCA-PSV exceeds 1.5 MoM, the clinical team initiates urgent evaluation for moderate to severe fetal anemia. The standard next step is cordocentesis (percutaneous umbilical blood sampling or PUBS), an invasive procedure where a needle is guided by ultrasound into the umbilical cord vein to obtain a fetal blood sample. This directly measures fetal hemoglobin, hematocrit, reticulocyte count, and blood type. If severe anemia is confirmed (fetal hematocrit below 30 percent), an intrauterine transfusion (IUT) is performed during the same procedure by infusing compatible packed red blood cells directly into the fetal circulation. IUT can be repeated every 2 to 4 weeks as needed until the fetus reaches a gestational age suitable for delivery, typically 34 to 37 weeks. After IUT, MCA-PSV interpretation becomes less reliable for several weeks because the transfused adult red blood cells alter fetal blood viscosity.
What are the limitations and potential errors in MCA Doppler assessment?
Several technical and biological factors can affect the accuracy of MCA-PSV measurements. The angle of insonation is critical, as measurements should be taken with the Doppler angle as close to zero degrees as possible; angles greater than 15 to 20 degrees introduce significant velocity overestimation. The sampling location matters, with measurements taken near the origin of the MCA from the internal carotid artery being most reproducible and reliable. Fetal behavioral state affects results, as active fetal movements or breathing movements can transiently elevate PSV values. After intrauterine transfusion, MCA-PSV loses predictive accuracy for 1 to 2 weeks because adult hemoglobin has different oxygen-carrying characteristics than fetal hemoglobin. After 35 weeks, sensitivity for detecting anemia decreases, and false-positive rates increase. Operator experience and equipment quality also contribute to measurement variability, emphasizing the importance of having trained sonographers perform these assessments.
Can MCA Doppler be used for conditions other than Rh disease?
MCA Doppler assessment has applications beyond Rh alloimmunization in several clinical scenarios involving fetal anemia or altered fetal hemodynamics. Parvovirus B19 infection monitoring uses MCA-PSV to detect fetal anemia from transient aplastic crisis, which can occur 4 to 6 weeks after maternal infection. In twin-to-twin transfusion syndrome (TTTS), MCA Doppler helps assess the donor twin for anemia and the recipient twin for polycythemia. Fetomaternal hemorrhage, which can cause acute or chronic fetal anemia, is monitored using serial MCA-PSV measurements. Some institutions also use MCA Doppler as part of the evaluation for fetal growth restriction, though its role in this context is primarily to assess cerebral blood flow redistribution (brain-sparing effect) using the cerebroplacental ratio rather than anemia detection. Research continues to explore additional applications including assessment of fetal polycythemia and prediction of neonatal outcomes.