Correct Answer: D. MCA doppler peak systolic velocity
In an Rh-negative sensitized mother (prior sensitization with anti-D antibodies), the primary concern is fetal hemolytic disease due to maternal IgG antibodies crossing the placenta. Modern obstetric practice has shifted from invasive testing to non-invasive Doppler assessment for fetal anemia surveillance. MCA (middle cerebral artery) peak systolic velocity (PSV) is the gold standard for detecting fetal anemia in sensitized pregnancies. An elevated MCA-PSV (>1.5 multiples of median [MoM] for gestational age) indicates compensatory increased cardiac output due to anemia and predicts moderate-to-severe fetal anemia with >90% sensitivity and specificity. This allows clinicians to identify fetuses requiring intrauterine transfusion (IUT) without the risks of amniocentesis or cordocentesis. The MCA-PSV measurement is performed via Doppler ultrasound at the level of the circle of Willis, measuring the peak systolic velocity in cm/s. This non-invasive approach has largely replaced older invasive methods in modern Indian obstetric practice, aligning with international guidelines (RCOG, ACOG) and reducing procedure-related losses in sensitized pregnancies.
Why the other options are wrong
A. Biophysical profile — BPP assesses fetal well-being through five parameters (NST, breathing, movement, tone, amniotic fluid volume) but is a late indicator of fetal distress. It does not specifically detect or quantify fetal anemia and is insensitive for hemolytic disease. BPP becomes abnormal only when severe fetal compromise has already occurred, making it unsuitable for early anemia detection in sensitized pregnancies where timely IUT is critical. B. Amniospectometry — Amniospectometry measures optical density of amniotic fluid at 450 nm wavelength to estimate bilirubin levels, historically used to assess hemolysis severity. However, it is indirect and non-specific—elevated OD450 reflects bilirubin, not anemia itself. It requires amniocentesis (invasive, 1% fetal loss risk) and cannot predict which fetuses need IUT. Modern practice has abandoned this in favor of direct Doppler assessment of fetal hemoglobin status. C. Fetal blood sampling — Cordocentesis (fetal blood sampling) directly measures fetal hemoglobin and hematocrit, providing definitive anemia assessment. However, it is invasive with 1–2% procedure-related loss risk and is now reserved only for therapeutic IUT or when MCA-PSV results are equivocal. In modern practice, MCA-PSV has replaced cordocentesis as the first-line surveillance tool, reserving invasive sampling only when intervention is planned.
High-Yield Facts
- MCA-PSV >1.5 MoM predicts moderate-to-severe fetal anemia with >90% sensitivity in Rh-sensitized pregnancies and guides timing of intrauterine transfusion.
- MCA-PSV is non-invasive, performed via Doppler ultrasound at the circle of Willis, eliminating procedure-related fetal loss compared to cordocentesis (1–2% risk).
- Amniospectometry (OD450) is now obsolete in modern obstetrics; it was indirect and required amniocentesis, which carries miscarriage risk.
- Biophysical profile is a late marker of fetal distress and does not detect anemia; it becomes abnormal only in severe, end-stage compromise.
- Intrauterine transfusion (IUT) is indicated when MCA-PSV is elevated or when cordocentesis confirms Hb <7 g/dL in a sensitized fetus.
Mnemonics
MCA-PSV in Sensitized Pregnancies Modern Cardiac Assessment = Peak Systolic Velocity. Non-invasive Doppler replaces invasive cordocentesis; >1.5 MoM = anemia, plan IUT. Why Not Amniospectometry? Amnio = Ancient. OD450 is historical; modern practice uses Doppler. Remember: amniospectometry = indirect bilirubin, not hemoglobin.
NBE Trap
NBE may lure candidates familiar with older obstetric literature toward amniospectometry or cordocentesis by framing them as "gold standard" investigations. The trap is not recognizing that modern non-invasive Doppler (MCA-PSV) has replaced invasive methods in current Indian and international obstetric practice, reducing fetal loss and improving outcomes in sensitized pregnancies.
Clinical Pearl
In Indian tertiary obstetric centers, a sensitized Rh-negative mother presenting at 18–20 weeks with elevated anti-D titers (>1:16) undergoes serial MCA-PSV Doppler every 1–2 weeks. If MCA-PSV crosses 1.5 MoM, cordocentesis is performed to confirm anemia and plan IUT, avoiding unnecessary invasive procedures in mildly affected fetuses and reducing perinatal mortality from 50% (untreated) to <5% (with timely IUT).
_Reference: DC Dutta's Textbook of Obstetrics (8th ed.), Ch. 18 (Rh Incompatibility); RCOG Green-top Guideline on Management of Pregnancy with Rhesus Negativity_