Rhesus alloimmunisation

Question 1

Causes

Answer 1

• Obstetric
  o Miscarriage, termination, ectopic pregnancy
  o CVS, amniocentesis, fetal blood sampling, ECV
  o Placental abruption, trauma, fetal death
  o Delivery
• Blood transfusion
• Grandmother effect – explanation for antibodies in primip (in-utero exposure of Rhneg infant to her mother’s Rhesus antigen)

Question 2

Prevention

Answer 2

• Anti-D
  o Routine administration has reduced risk from 16% (ABO compatible infant) or 2% (ABO incompatible infant) to <0.5%
  o Given routinely at 28 and 34/40, and clinically indicated in cases of above causes
  o First trimester – 250IU, second/third trimesters – 625IU

Question 3

Important features to note in history

Answer 3

• Possible exposures
• Previous affected pregnancy details including:
• IUT
• Titres
• GA at delivery
• Jaundice

Question 4

Investigations

Answer 4

o Blood group and antibody titre
- Critical titre is 1:32
- Titres are related to fetal risk (1:512=50% risk of anaemia)
- If the woman has had a previously affected pregnancy, titres are irrelevant and fetal surveillance should be performed regardless
Paternal blood type
- If rhesus negative and paternity certain, baby not at risk
- If rhesus positive, genotype required
* 60% will be heterozygous and only have 50% chance of rhesus positive offspring
* If homozygous, fetus is affected so no further invasive testing is required to determine fetal type
o Fetal blood type
- May be considered if paternal heterozygosity or uncertain paternity
- Can be determined using CVS/amniocentesis
- Only required for patients in whom surveillance will be difficult (ie live remotely)

Question 5

Management

Answer 5

• Titre <1:32 and no previous affected pregnancy
  o Repeat antibody titre 4 weekly, consider fortnightly from 28-32/40
  o Antibody reaching critical titre should prompt MFM referral
• Titre >=1:32
  o Tertiary centre
  o Commence 1-2 weekly MCA PSVs (from 16/40)
  o 1.5MoMs is significant (100% sensitivity, 12% false positive)
  o If abnormal MCA PSV, proceed for fetal blood sampling (cordocentesis) +/- intrauterine transfusion (IUT)
• Steroids prior if viable
• Direct ultrasound guidance with aseptic technique
• Local anaesthetic
• Spinal needle to umbilical vein to sample blood
• Haemacue used to rapidly assess fetal haemoglobin
• Complications
• 1.4% fetal loss
• 1% PPROM, infection
• 1% requiring immediate delivery (cord spasm, abruption)
• 20-30% cord bleeding
• 5-10% fetal bradycardia
  o IUT
• Transfusion of red cells into umbilical vein
• Indicated if haematocrit <30%
• Transfuse O-ve blood, CMV –ve, irradiated, leucocyte deplete, Hct 80, cross-matched to mother
• Dose calculated based on EFW and Hct aiming for post-transfusion Hct 40-50%
• Complications
• Old data
  o 9% complication rate
  o 3% fetal loss, 2% neonatal death
  o 6% need for emergency CS
  o 1% infection
• New data (937 IUTs after 2001)
  o 1.2% per procedure complication rate
  o 0.4% emergency CS
  o 0.6% fetal loss
  o 0.1% infection/PPROM
  o CTG surveillance from 35/40
  o Delivery at term (37-38/40)
• Vaginal birth unless fetal distress
• CEFM during labour
• Paediatric attendance if preterm, IUT, delivery for fetal indications
• Cord blood sent for FBE, bilirubin, reticulocytes, Hb, Coombes
  o Anti-D not required post-partum