MCA doppler during NT screening

Role of MCA doppler

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MCA Doppler is not a standard or mandatory component of first-trimester NT screening. NT screening mainly includes:

• Nuchal Translucency (NT) measurement
• Maternal serum markers (PAPP-A, free β-hCG)
• Optional Doppler studies: Ductus venosus and Uterine artery

MCA Doppler is generally not required unless the fetus is at high risk or there's a clinical suspicion based on other findings.

MCA Doppler may be considered in selected high-risk situations:

• History of fetal anemia or hydrops
• Suspected early-onset IUGR or placental insufficiency
• Chromosomal or cardiac anomalies suggested by NT, DV, or tricuspid flow

Even in these cases, it is usually used in **research** or **specialized fetal medicine centers**, not general NT screening.

Patient Preparation

No specific preparation is needed. A partially full bladder may assist in optimal fetal visualization. The fetus should be calm, as excessive movement can interfere with waveform accuracy.

Technique

A transabdominal approach is used. The fetal head is visualized in a transverse axial plane at the level of the thalami. Color Doppler is applied to identify the Circle of Willis. Pulsed Doppler sampling is done near the origin of the MCA (within 5 mm), avoiding a Doppler angle >15°.

Normal Doppler Values during NT Screening (11–13+6 weeks)

• PI (Pulsatility Index): 1.4 – 1.9
• RI (Resistance Index): 0.75 – 0.85
• PSV (Peak Systolic Velocity): 20 – 35 cm/s
• A-wave: Always positive (forward flow)

Clinical Interpretation of Abnormal MCA Doppler

High PI: Suggests increased cerebral resistance, possibly indicating early placental insufficiency or early IUGR.
Low PI: May reflect "brain-sparing effect" if fetus is under hypoxic stress, even this early. Requires correlation with other markers.
High RI: Indicates reduced diastolic flow in the cerebral artery; may be associated with elevated vascular resistance.
Low RI: Can be physiological in early gestation or suggest vasodilation due to fetal adaptation in high-risk pregnancies.
High PSV: May suggest fetal anemia (though uncommon during NT scan) or AV malformations. Evaluate in the context of MCA/placenta perfusion.
Low PSV: May suggest fetal hypoxia or growth compromise. Correlate with other Doppler findings.
Absent or Reversed A-wave: Highly abnormal finding suggesting increased fetal central venous pressure. Rare at NT scan, but if present, may indicate chromosomal anomalies or cardiac dysfunction.

Clinical Use

MCA Doppler in the first trimester is not routinely used but may offer added value in high-risk pregnancies. When used with uterine artery Doppler, NT, DV flow, and serum markers, it can assist in early detection of placental dysfunction or high-risk fetal conditions.

Note:

Interpret MCA Doppler in combination with NT thickness, ductus venosus waveform, uterine artery PI, PAPP-A, and maternal risk factors. Isolated abnormalities should not guide management without comprehensive assessment.