Is middle cerebral artery (MCA) Doppler assessment limited to after 32 weeks of gestation?

Middle Cerebral Artery Doppler Assessment in Fetal Growth Restriction

Middle cerebral artery (MCA) Doppler assessment is primarily recommended after 32 weeks of gestation because it is most clinically relevant for late-onset fetal growth restriction (FGR), which is defined as FGR diagnosed at or after 32 weeks. 1

Why MCA Doppler is Recommended After 32 Weeks

• MCA Doppler assessment is specifically indicated for late-onset FGR (≥32 weeks) as part of the surveillance protocol for these pregnancies 1
• In late-onset FGR, cerebral blood flow redistribution ("brain-sparing effect") becomes a more significant marker of fetal adaptation to placental insufficiency 1
• Multiple national guidelines, including those from the United Kingdom, recommend MCA Doppler specifically after 32 weeks with normal umbilical artery (UA) Doppler 1

Physiological Basis

• MCA Doppler patterns change throughout gestation, with MCA pulsatility index (PI) increasing from 20 weeks to reach a peak around 32 weeks, then decreasing thereafter 2
• The cerebroplacental ratio (CPR), which incorporates MCA Doppler, follows a similar pattern, peaking around 34 weeks 2
• Before 32 weeks, umbilical artery Doppler is the primary tool for monitoring early-onset FGR, as it better reflects placental resistance in earlier gestation 1

Clinical Application in Different Types of FGR

Early-onset FGR (<32 weeks):

• Primary monitoring relies on umbilical artery Doppler assessment 1
• MCA Doppler is not routinely recommended for clinical management of early-onset FGR 1, 3
• The Society for Maternal-Fetal Medicine (SMFM) suggests that "Doppler assessment of the ductus venosus, middle cerebral artery, or uterine artery not be used for routine clinical management of early-onset FGR" 1

Late-onset FGR (≥32 weeks):

• MCA Doppler becomes more valuable as a surveillance tool 1
• Reduced MCA PI (<5th percentile) indicates cerebral blood flow redistribution and may influence delivery timing 1
• UK guidelines recommend delivery by 37 weeks if MCA PI <5th centile 1

Surveillance Protocols by Gestational Age

• For FGR with EFW ≥3rd-9th percentile: UA Doppler every 1-2 weeks initially, then every 2-4 weeks if stable 1
• For FGR with EFW <3rd percentile: UA Doppler weekly 1
• After 32 weeks: Add MCA Doppler assessment to the surveillance protocol 1
• New Zealand guidelines recommend MCA Doppler and CPR every 2 weeks after 34 weeks 1

Clinical Significance and Delivery Timing

• Abnormal MCA Doppler (PI <5th centile) after 32 weeks may indicate the need for delivery by 37 weeks 1
• In small fetuses with normal umbilical artery Doppler, MCA redistribution is associated with increased rates of cesarean delivery and neonatal admission 4
• An elevated head circumference/abdominal circumference ratio strongly correlates with MCA blood flow redistribution 4

Limitations and Pitfalls

• MCA Doppler has limited sensitivity (69%) for detecting severe fetal compromise when used alone 3
• False-positive MCA Doppler measurements can occur, particularly in fetuses with previous intrauterine transfusions 3
• The impact of MCA Doppler on outcomes in early FGR (<32 weeks) is modest and less significant than birthweight and delivery gestation 5
• Longitudinal assessment of MCA Doppler trends may provide more clinical information than single measurements 6

In summary, MCA Doppler assessment is primarily recommended after 32 weeks because it has greater clinical utility in late-onset FGR, aligns with the physiological changes in cerebral blood flow that occur in the third trimester, and forms part of established surveillance protocols for pregnancies affected by FGR after 32 weeks.