How to manage an Rh negative pregnancy at approximately 32 weeks gestation with an antibody titre of 1:512 and a Middle Cerebral Artery Peak Systolic Velocity (MCA PSV) Multiple of Median (MoM) of 1.46?

Management of Rh-Negative Pregnancy at 32 Weeks with Antibody Titer 1:512 and MCA PSV MoM 1.46

Continue weekly MCA PSV surveillance without immediate intervention, as the MCA PSV of 1.46 MoM does not meet the threshold for severe fetal anemia requiring intrauterine transfusion. 1

Current Risk Assessment

Your patient's MCA PSV of 1.46 MoM falls below the critical threshold of 1.5 MoM that defines significant risk for severe fetal anemia requiring intervention. 1 While the antibody titer of 1:512 is markedly elevated and indicates high-risk pregnancy, the MCA PSV is the primary determinant for timing of intervention, not the antibody titer alone. 1

According to the Society for Maternal-Fetal Medicine guidelines:

• MCA PSV >1.5 MoM indicates significant risk for severe fetal anemia and warrants consideration for fetal blood sampling with preparation for intrauterine transfusion (IUT). 1
• MCA PSV values between 1.0-1.5 MoM represent mild to moderate anemia risk but do not typically require immediate invasive intervention. 1

Immediate Management Plan

Weekly MCA PSV Monitoring

• Perform MCA PSV measurements weekly given the high antibody titer and gestational age of 32 weeks. 1, 2
• Ensure proper technique: measure the middle cerebral artery close to its origin, ideally at zero-degree angle without angle correction to avoid overestimation. 1
• If MCA PSV rises above 1.5 MoM, immediately refer for fetal blood sampling with preparation for IUT. 1

Ultrasound Surveillance

• Assess for signs of fetal hydrops at each visit (ascites, pleural effusion, pericardial effusion, skin edema, polyhydramnios). 1
• Presence of hydrops is an indication for immediate fetal blood sampling regardless of MCA PSV value. 1
• Monitor for cardiomegaly and abnormal Doppler findings in umbilical artery, umbilical vein, or ductus venosus. 1

Referral Considerations

Refer to a tertiary care center with expertise in invasive fetal therapy given the high antibody titer and need for potential rapid intervention. 1 This ensures immediate access to:

• Fetal blood sampling (cordocentesis) capabilities
• Intrauterine transfusion expertise
• Neonatal intensive care unit for delivery management

Delivery Planning

• Plan delivery at 37-38 weeks gestation unless complications develop requiring earlier intervention. 1
• Earlier delivery may be indicated if:
  • MCA PSV exceeds 1.5 MoM and gestational age makes delivery safer than IUT (typically >34-35 weeks)
  • Fetal hydrops develops
  • Other obstetric indications arise

Critical Thresholds to Monitor

Intervention Triggers

• MCA PSV >1.5 MoM: Proceed to fetal blood sampling with IUT preparation. 1
• Any signs of hydrops: Immediate fetal blood sampling indicated. 1
• Abnormal Doppler velocimetry in umbilical artery, umbilical vein, or ductus venosus suggests severe disease. 1

Hemoglobin Thresholds (if fetal blood sampling performed)

At 32 weeks gestation, based on reference ranges: 1

• Normal hemoglobin: ≥11.0 g/dL (≥0.84 MoM)
• Mild anemia: 8.5-11.0 g/dL (0.65-0.84 MoM)
• Moderate anemia: 7.2-8.5 g/dL (0.55-0.64 MoM)
• Severe anemia: <7.2 g/dL (<0.55 MoM) - requires IUT

Important Caveats

Approximately 10% false-positive rate exists with MCA PSV assessments, meaning some fetuses with elevated MCA PSV may not be severely anemic. 1 However, given the high antibody titer, err on the side of intervention if MCA PSV exceeds 1.5 MoM rather than waiting for confirmatory signs. 1

Adverse perinatal outcomes correlate with:

• MCA PSV >1.5 MoM (p<0.001)
• Presence of hydrops (p<0.001)
• Early gestational age at first transfusion (p=0.029)

2

Therefore, close surveillance with weekly MCA PSV is essential to detect progression before hydrops develops, as outcomes are significantly better with earlier detection and intervention. 2