Rhesus Isoimmunisation
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Rhesus Isoimmunisation occurs when a Rhesus Negative (RhD -ve) mother carries a Rhesus Positive (RhD +ve) fetus. Exposure to fetal blood causes the mother to produce antibodies (Anti-D) against the fetal red blood cells. These antibodies cross the placenta, causing Haemolytic Disease of the Fetus and Newborn (HDFN).
Clinical Scenario: The Trauma Call
A 28-week pregnant woman (RhD Negative) is involved in an RTA (steering wheel injury). She feels fine. CTG is normal.
Key Teaching Points
- **Risk**: 'Silent' Fetomaternal Haemorrhage (FMH). Even minor trauma can cause fetal blood to leak into maternal circulation.
- **Action**: She needs a **Kleihauer Test** (to quantify the fetal bleed) and a dose of **Anti-D Immunoglobulin**.
- **Kleihauer**: If the bleed is large (>4ml), the standard dose of Anti-D (500iu or 1500iu) may not be enough, and extra Anti-D is required to 'mop up' all the fetal cells.
Image Integration Plan
| Image Type | Source | Status |
|---|---|---|
| Management Algorithm | AI-generated | PENDING |
| MCA Doppler Waveform | Web Source | PENDING |
| Diagram (Sensitisation) | AI-generated | PENDING |
| Kleihauer Slide | Web Source | PENDING |
[!NOTE] Image Generation Status: Diagrams illustrating the placental transfer of IgG are queued.
Timeline of Events
- Sensitising Event: Fetal cells enter maternal blood.
- Sensitisation: Maternal immune system makes IgM (short term) then memory B cells.
- Next Pregnancy: Maternal Memory B cells produce IgG (small enough to cross placenta).
- Haemolysis: IgG attacks fetal RBCs -> Anaemia -> Hydrops.
- Prevalence: ~15% of Caucasian women are RhD Negative.
- Success: Routine Anti-D prophylaxis has reduced sensitisation from 16% to <1%.
- Rh System: The D antigen is highly immunogenic.
- HDFN:
- Maternal Alloantibodies (Anti-D, Anti-c, Anti-Kell) coat fetal RBCs.
- RBCs destroyed by fetal spleen (extravascular haemolysis).
- Anaemia: Fetal bone marrow and Liver (extramedullary haematopoiesis) go into overdrive. Liver enlarges -> Portal hypertension -> Ascites.
- Hydrops Fetalis: Severe anaemia causes high output heart failure + low oncotic pressure (liver failure) -> Oedema, Pleural effusion, Ascites.
Maternal
Fetal
Neonatal
- Maternal Booking Bloods (12 weeks):
- ABO and RhD Group.
- Antibody Screen (Indirect Coombs).
- Antibody Monitoring:
- If antibodies detected, quantify levels (Titres).
- Rising titres indicate active haemolysis.
- Fetal Monitoring:
- MCA Doppler (Middle Cerebral Artery):
- Anaemic blood is "thinner" (less viscous) -> flows faster.
- Peak Systolic Velocity (PSV) > 1.5 MoM = Moderate/Severe Anaemia.
- Kleihauer-Betke Test:
- Acid elution test on maternal blood.
- Adult HbA is eluted (cells look like ghosts). Fetal HbF resists acid (cells look pink).
- Used to calculate volume of FMH.
Routine Antenatal Anti-D Prophylaxis (RAADP):
- Given to all non-sensitised RhD Negative women.
- Single dose at 28 weeks OR Two doses (28 and 34 weeks).
Additional Anti-D: Given within 72 hours of any Sensitising Event:
- Miscarriage (>12 weeks) / Termination / Ectopic.
- Antepartum Haemorrhage (APH).
- Abdominal Trauma.
- Invasive testing (Amniocentesis / CVS).
- External Cephalic Version (ECV).
- Delivery (if baby is RhD Positive).
Anti-D is useless if the woman is already sensitised (it cannot reverse antibodies).
- Fetal Surveillance: Weekly MCA Dopplers.
- Intrauterine Transfusion (IUT):
- Indication: Severe Anaemia / Hydrops < 34 weeks.
- Procedure: O-Negative blood injected into umbilical vein (cordocentesis).
- Delivery:
- Early delivery (37-38 weeks) to minimise exposure.
- Immediate delivery if fetal distress.
- Phototherapy: Blue light converts bilirubin to water-soluble isomer.
- Exchange Transfusion:
- If bilirubin rising rapidly despite lights.
- Removes bilirubin AND circulating maternal antibodies.
- IVIG: Can block haemolysis.
- Fetal Death (Stillbirth).
- Kernicterus: Permanent brain damage (Deafness, CP) from bilirubin.
- RCOG Green-top Guideline 65: Rhesus D Prophylaxis.
- NICE Guidelines: Antenatal Care.
What is Rhesus Disease? Blood types have a "Positive" or "Negative" tag (like O Positive). If you are Negative and your baby is Positive (inherited from the father), your body might see the baby's blood as an "invader". If your blood mixes (e.g., during birth or a fall), your immune system makes weapons (antibodies) to attack the baby's blood.
Does it affect the first pregnancy? Usually, no. It takes time to make the weapons. The danger is for the next pregnancy. The weapons are ready and waiting, and can attack the new baby even without any bleeding.
How do we stop it? We give you an injection called Anti-D. Think of it as a "clean-up crew". If any of the baby's blood gets into your system, the injection cleans it up before your immune system notices it. This stops you from ever making the weapons.
Why do I need the Kleihauer test? This test checks exactly how much baby blood has leaked into yours. If it was a big leak (like after a car crash), one injection might not be enough, and we need to calculate a bigger dose.
- Qureshi H, et al. BCSH Guideline for the use of anti-D immunoglobulin for the prevention of HDFN. Transfus Med. 2014.
- RCOG. The Management of Women with Red Cell Antibodies during Pregnancy (Green-top Guideline No. 65). 2014.
- Moise KJ. Management of Rhesus Alloimmunization in Pregnancy. Obstet Gynecol. 2008.