Rh Isoimmunisation
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Direct answer
Rh isoimmunisation occurs when an Rh-negative mother forms anti-D antibodies after fetomaternal haemorrhage from an Rh-positive fetus; IgG antibodies then cross the placenta in subsequent pregnancies to haemolyse fetal red cells, causing anaemia, hydrops and stillbirth. Prevention with anti-D immunoglobulin — 300 micrograms at about 28 weeks and within 72 hours of delivery of an Rh-positive baby, plus after sensitising events — has made the disease rare; an alloimmunised pregnancy is monitored with titres and middle cerebral artery Doppler, with intrauterine transfusion for significant anaemia and timed delivery.
What you must remember
- Sensitising events: childbirth, abortion, ectopic pregnancy, amniocentesis or chorionic villus sampling, antepartum haemorrhage, external cephalic version, abdominal trauma and intrauterine death — any event pushing fetal blood into the maternal circulation.
- Routine prophylaxis: anti-D 300 micrograms (1500 IU) intramuscularly at about 28 weeks to unsensitised Rh-negative women, repeated within 72 hours of delivery if the baby is Rh-positive; smaller doses cover early first-trimester events per several guidelines.
- Fetomaternal haemorrhage quantification: the Kleihauer-Betke acid elution test (or flow cytometry) after delivery or large sensitising events; 300 micrograms covers about 30 ml of fetal whole blood, extra doses if exceeded.
- Screening and titres: blood group and indirect Coombs test at booking and around 28 weeks; a critical titre of about 1:16 (some laboratories 1:32) triggers fetal surveillance rather than serial titres alone.
- Fetal surveillance: middle cerebral artery peak systolic velocity — values above 1.5 multiples of the median predict moderate to severe anaemia; amniotic fluid bilirubin by spectrophotometry (delta OD450, Liley or Queenan charts) is the traditional alternative; highly sensitised women are tested every 1-2 weeks from about 16-20 weeks.
- Treatment of the affected fetus: intrauterine intravascular transfusion of O-negative packed cells into the umbilical vein under ultrasound, repeated every 2-4 weeks until delivery (usually 34-37 weeks); hydrops fetalis — ascites, skin oedema, effusions — indicates advanced disease; neonatal care includes phototherapy, immunoglobulin and exchange transfusion.
- ABO versus Rh: ABO incompatibility (typically mother O, baby A or B) causes milder haemolysis from IgM that crosses poorly, may affect a first pregnancy and needs no prophylaxis; Rh disease worsens with successive pregnancies as IgG memory amplifies.
Common confusion
The preventable window is misunderstood: the first Rh-positive pregnancy usually sensitises at delivery rather than damaging that fetus, so without prophylaxis the damage lands on subsequent pregnancies — the rationale for postnatal anti-D within 72 hours. Once the mother is immunised (positive indirect Coombs), prophylaxis is useless and she needs fetal surveillance; a titre is a triage tool, with Doppler assessment beginning only beyond the critical value.
Exam-focused takeaway
Stems test the prophylaxis schedule (28 weeks plus 72 hours postnatally), the Kleihauer quantification of fetomaternal haemorrhage and the 1.5 MoM middle cerebral artery threshold. Expect matching on Liley zones versus Doppler, indications for intrauterine transfusion, the hydrops picture and the ABO-versus-Rh comparison.
Frequently asked questions
When is routine anti-D given?
300 micrograms intramuscularly at about 28 weeks to unsensitised Rh-negative women, repeated within 72 hours of delivery if the newborn is Rh-positive.
What is a critical antibody titre?
About 1:16 (or 1:32 by laboratory), beyond which middle cerebral artery Doppler surveillance replaces titre follow-up.
How is fetal anaemia detected non-invasively?
Middle cerebral artery peak systolic velocity above 1.5 multiples of the median predicts moderate to severe anaemia non-invasively.
What is intrauterine transfusion?
Ultrasound-guided infusion of O-negative packed red cells into the umbilical vein, repeated every few weeks until delivery in significantly anaemic fetuses.
Why is ABO incompatibility milder than Rh disease?
ABO haemolysis involves mostly IgM antibodies that cross the placenta poorly and does not amplify with parity, whereas Rh IgG memory antibodies cross readily and worsen in successive pregnancies.