What is the significance of an abnormal cerebroplacental ratio (CPR) in the absence of umbilical artery Doppler abnormalities?

Abnormal CPR with Normal Umbilical Artery Doppler

An abnormal cerebroplacental ratio (CPR) in the absence of umbilical artery Doppler abnormalities indicates early fetal cerebral vasodilation (brain-sparing effect) that precedes detectable placental resistance changes, representing a more sensitive marker of placental insufficiency and fetal compromise than umbilical artery Doppler alone. 1

Physiologic Mechanism

The CPR becomes abnormal through two distinct pathways that can occur independently:

• Decreased middle cerebral artery (MCA) pulsatility index reflects cerebral vasodilation in response to fetal hypoxemia, representing the brain-sparing effect 1
• This adaptive mechanism can occur before umbilical artery resistance increases sufficiently to produce abnormal Doppler indices, making CPR an earlier predictor of fetal compromise 1
• The fetus redistributes blood flow to protect vital organs (brain, heart, adrenals) even when placental function is only mildly impaired 1

Clinical Significance as Early Warning Sign

CPR may be an earlier predictor of adverse outcome than umbilical artery Doppler abnormalities alone:

• CPR detects fetal adaptation before umbilical artery changes become apparent, providing a window for intervention 1
• Studies demonstrate that CPR abnormalities precede biophysical profile changes and umbilical artery Doppler abnormalities in the progression of fetal compromise 1, 2
• An abnormal CPR identifies fetuses at risk even when individual MCA and umbilical artery measurements appear normal in isolation 2

Risk Stratification by Fetal Size

The clinical implications differ based on estimated fetal weight:

Appropriate for Gestational Age (AGA) Fetuses with Abnormal CPR

• These fetuses represent "failure to reach growth potential" (FRGP) - they are constitutionally larger fetuses experiencing placental insufficiency but remaining above the 10th percentile 3
• AGA fetuses with abnormal CPR have significantly higher rates of intrapartum fetal compromise, emergency cesarean delivery for fetal distress, lower cord pH, and increased NICU admissions compared to those with normal CPR 2, 4
• The cut-off value of CPR <1.49 in AGA fetuses predicts adverse outcomes with 67.5% sensitivity and 68% specificity 4
• Delivery at 39 weeks gestation is recommended for AGA fetuses with abnormal CPR based on risk stratification studies 3

Small for Gestational Age (SGA)/FGR Fetuses with Abnormal CPR

• Fetuses with FGR and abnormal CPR demonstrate higher rates of: lower gestational age at birth, lower birthweight, cesarean delivery for fetal distress, Apgar scores <7 at 5 minutes, neonatal acidosis, NICU admissions, and perinatal death compared to FGR with normal CPR 2
• Abnormal CPR in FGR is associated with significantly poorer neurodevelopmental outcomes at 3 years of age across all measured variables compared to SGA with normal Doppler or FGR with normal CPR 5
• The combination of abnormal CPR with abnormal umbilical artery PI increases risk of cesarean delivery for non-reassuring fetal status 7-fold (adjusted OR 7.0,95% CI 1.2-41.3) 6

Surveillance Recommendations

When CPR is abnormal but umbilical artery Doppler remains normal:

• Repeat umbilical artery Doppler every 1-2 weeks initially to monitor for progression 1
• If umbilical artery Doppler remains stable after initial assessment, extend interval to every 2-4 weeks 1
• Include weekly cardiotocography (CTG) as part of fetal surveillance 1
• Assess fetal growth every 2-3 weeks (can consider 2-week intervals for severe cases) 1
• Monitor amniotic fluid volume as a marker of chronic placental insufficiency 1

Important Clinical Caveats

Limitations of CPR as standalone test:

• Current guidelines state there is insufficient evidence to use CPR as a stand-alone test for clinical decision-making 1
• CPR should be incorporated as a component of comprehensive fetal surveillance, not used in isolation 1
• Randomized controlled trials are needed to establish optimal management protocols based on CPR values 1

Practical considerations:

• The ACR recommends reporting pulsatility index (rather than S/D ratio) specifically to facilitate CPR calculation 1
• CPR assessment should be considered in all fetuses undergoing third-trimester ultrasound, regardless of individual MCA or umbilical artery values 2
• The presence of abnormal CPR warrants increased surveillance intensity even when umbilical artery Doppler appears reassuring 1, 2