Rh Haemolytic Disease of the Newborn
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Direct answer
Rh haemolytic disease of the newborn is haemolysis of fetal red cells by maternal IgG anti-D antibodies, usually after an Rh-negative mother has been sensitised by an earlier Rh-positive pregnancy. The antibodies cross the placenta, causing anaemia, jaundice and, in severe cases, hydrops fetalis. It is prevented by anti-D immunoglobulin at 28 weeks of pregnancy and after any sensitising event, including delivery of an Rh-positive baby.
What you must remember
- Mechanism: maternal IgG anti-D, produced after sensitisation, crosses the placenta and binds D-positive fetal red cells, which are then removed in the spleen.
- Sensitising events: feto-maternal haemorrhage at delivery, abortion, ectopic pregnancy, antepartum haemorrhage, amniocentesis and abdominal trauma.
- Antenatal screening: an antibody screen by indirect Coombs test is done in every pregnancy, with the titre followed in sensitised women.
- Neonatal features: early jaundice, anaemia, hepatosplenomegaly and, in severe disease, hydrops fetalis.
- Direct Coombs test: positive in the newborn, confirming antibody coating the infant's red cells.
- Prophylaxis: anti-D immunoglobulin 300 micrograms at 28 weeks and within 72 hours after delivery of an Rh-positive baby.
- Intrauterine treatment: intrauterine transfusion for severe fetal anaemia, guided by middle cerebral artery Doppler.
A pathway from screening to neonatal care
Rh disease is managed as a sequence from antenatal screening to neonatal treatment, and the steps are a common examination topic. Each step responds to the findings of the previous one.
- Screening: Every pregnant woman has a blood group and antibody screen, and a positive indirect Coombs test identifies a sensitised mother.
- Assessment: Fetal anaemia is assessed by middle cerebral artery peak systolic velocity Doppler, whose rise indicates a falling haemoglobin.
- Intrauterine transfusion: Severe fetal anaemia is treated by intrauterine transfusion, usually through the umbilical cord, when the fetus is at a suitable gestation.
- Timed delivery: The obstetric team plans delivery when fetal maturity and condition allow, balancing the risks of prematurity against those of continued disease.
- Neonatal management: The newborn undergoes direct Coombs testing and monitoring of haemoglobin and bilirubin, with phototherapy or exchange transfusion as indicated.
- Prophylaxis for future pregnancies: Anti-D is given after each Rh-positive delivery and any sensitising event in an Rh-negative woman.
Because the sensitised mother carries the risk into later pregnancies, the steps are repeated for each subsequent pregnancy.
Where the exam tries to catch you
Rh disease stems usually test the timing of sensitisation, the two Coombs tests and the anti-D schedule.
- Trap: expecting the first Rh-positive baby to be severely affected. Correct: the first pregnancy is usually spared, because sensitisation occurs at delivery, and severity rises with later pregnancies.
- Trap: confusing the indirect and direct Coombs tests. Correct: the indirect test detects antibody in maternal serum, while the direct test detects antibody coating the newborn's red cells.
- Trap: giving anti-D only after the baby is born. Correct: antenatal anti-D at 28 weeks prevents sensitisation during pregnancy, and a postnatal dose covers the delivery.
Frequently asked questions
Why is the first Rh-positive baby usually not affected by Rh disease?
The first Rh-positive baby is usually not affected by Rh disease, because maternal sensitisation mostly occurs during or after delivery rather than before it. The antibodies formed after the first pregnancy then threaten later Rh-positive babies.
At what gestation is anti-D immunoglobulin given in Rh-negative pregnancy?
Anti-D immunoglobulin is given to Rh-negative pregnant women at 28 weeks of gestation as routine antenatal prophylaxis. A further dose is given within 72 hours after delivery of an Rh-positive baby.
What is the difference between the direct and indirect Coombs test?
The indirect Coombs test detects free antibody in maternal serum and is used for antenatal screening. The direct Coombs test detects antibody coating the newborn's red cells and confirms immune haemolysis after birth.
Which Doppler test assesses fetal anaemia in Rh disease?
Middle cerebral artery peak systolic velocity Doppler assesses fetal anaemia in Rh disease, because its velocity rises as the fetal haemoglobin falls. It is used to guide intrauterine transfusion in place of repeated amniocentesis.