Normal Middle Cerebral Artery Doppler Values at 28 Weeks Gestation
At 28 weeks gestation, the normal pulsatility index (PI) for the fetal middle cerebral artery is approximately 1.9, and the resistance index (RI) is approximately 0.8 1.
Specific Reference Values
Based on the highest quality research data available:
These values are derived from a large prospective study of 962 low-risk pregnancies that established reference curves using automated waveform analysis 1.
Understanding the Normal Pattern
The MCA Doppler indices follow a parabolic pattern throughout pregnancy:
- Values increase from 18 weeks (PI: 1.5, RI: 0.68) to peak around 28-30 weeks
- Then decrease toward term at 42 weeks (PI: 1.1, RI: 0.61) 1
This parabolic curve has been consistently demonstrated across multiple studies 2, 3, 4, 5, with the turning point occurring precisely around your gestational age of interest.
Technical Considerations for Accurate Measurement
When obtaining MCA Doppler measurements 6:
- Measure at the proximal portion of the vessel immediately near the circle of Willis (this location shows best reproducibility)
- Use a transverse plane at the base of the fetal skull
- Maintain angle of incidence at 0 degrees (angle correction is not necessary for PI measurement)
- Avoid measurements during fetal breathing movements
Clinical Significance
Abnormal findings are defined as increased diastolic flow (decreased PI/RI values below the 5th percentile for gestational age), which indicates the "brain-sparing reflex" 6. This cerebral vasodilation occurs in response to fetal hypoxemia and is associated with:
- Neonatal acidosis
- Neurological impairment
- Adverse perinatal outcomes
At 28 weeks, a PI value significantly below 1.9 or RI below 0.8 would suggest cerebral blood flow redistribution and warrants further evaluation for fetal compromise 7.
Important Caveat
The location of measurement matters: The proximal MCA has lower PI values (1.61) compared to mid (1.77) and distal (1.84) portions of the vessel 2. Always measure at the proximal location for consistency and comparison to reference ranges.