Rhesus alloimmunisation

• Rhesus alloimmunisation means an Rh-negative woman has made antibodies (anti-D) against the RhD antigen on her baby's red cells, usually after an earlier pregnancy, delivery, or bleeding event.
• In a first pregnancy the mother's exposure usually causes no illness in that fetus, but it sensitises her for life; in the next Rh-positive pregnancy her IgG anti-D crosses the placenta, destroys fetal red cells, and can cause haemolytic disease of the fetus and newborn (HDFN).
• Routine RhD typing and antibody screening at booking and at 28 weeks, together with targeted anti-D immunoglobulin after sensitising events and as routine antenatal prophylaxis, prevent sensitisation in almost all cases.
• An affected pregnancy is managed by a fetal medicine specialist who tracks maternal antibody levels, performs MCA Doppler ultrasound, and, if needed, gives intrauterine transfusions.
• Complications of untreated disease (fetal anaemia, hydrops, kernicterus, stillbirth) are now rare where prophylaxis programmes exist, but remain common in countries without them.

By the end of this page you should be able to:

1. Explain what RhD is and how a sensitising event in one pregnancy leads to haemolysis in the next.
2. State who needs anti-D prophylaxis, at what dose and timing, and after which events.
3. Distinguish passive from immune anti-D on a blood result, and list the monitoring thresholds for immune anti-D.
4. Plan the investigation of a sensitised pregnancy, including the role of the Kleihauer-Betke test, MCA Doppler and intrauterine transfusion.
5. Compare the recommendations of RCOG, ACOG, NICE and WHO on routine anti-D prophylaxis.
6. Tell RhD alloimmunisation apart from ABO incompatibility, other red cell antibodies and non-immune causes of fetal anaemia.

The RhD antigen is part of the Rh blood group system on the red cell membrane. People who carry it are RhD positive; people who lack it are RhD negative. A mother is only at risk if she is RhD negative and carries a fetus who is RhD positive, which depends on paternal inheritance (HaemBase notes that 60% of RhD-negative mothers carry an RhD-positive baby). ^[8]

Classification that matters in practice falls into three groups:

• Unsensitised RhD negative: no anti-D; eligible for prophylaxis.
• Passively immunised: anti-D has been administered and is detectable in the circulation; she still needs prophylaxis after further sensitising events until immunity is disproven.
• Alloimmunised (immune): anti-D is present from prior exposure, does not fall with time, and confers risk on the current and future RhD-positive fetuses.

Anti-D disease should also be separated from other red cell antibodies. Broadly, antibodies associated with clinically significant HDFN include anti-D, anti-c and anti-K; many other antibodies usually cause only neonatal rather than fetal disease. ^[8]

People who type as weak D or partial D are managed differently by laboratory policy; in most programmes, some variants (for example some weak D types) are treated as RhD positive and not given prophylaxis, while others are treated as RhD negative. The exact policy should be confirmed with the local transfusion laboratory rather than assumed.

RhD negativity varies markedly by ethnicity: about 15% of people in the UK and about 15% in North America, 5% in India, and 3-4% in China are RhD negative. ^{[2][1] [8]}

The burden of disease depends almost entirely on whether a prophylaxis programme exists:

• Without prophylaxis, roughly 13-16% of at-risk (RhD-negative mother, RhD-positive fetus) pregnancies become alloimmunised. ^[2]
• In countries without prophylaxis, 14% of affected fetuses are stillborn and about half of live-born infants with haemolytic disease suffer neonatal death or brain injury. ^[2]
• With postpartum prophylaxis alone, alloimmunisation fell from 13-16% to 0.5-1.8% of at-risk pregnancies; adding routine antenatal prophylaxis reduced it further to 0.14-0.2%. ^{[2] [1][9]}
• Even in the USA, where prophylaxis is routine, alloimmunisation occurs in well under 1% of at-risk pregnancies, and about 1% of pregnant women overall have a clinically significant red cell antibody (anti-D is still the commonest single specificity). ^[1][8]
• Even with prophylaxis, alloimmunisation has not been abolished; sensitisation is usually due to failure to follow protocol, occult feto-maternal haemorrhage before 28 weeks, or prior transfusion of RhD-positive blood rather than prophylaxis failure. ^[9]

The only prerequisite for RhD alloimmunisation is an RhD-negative mother exposed to RhD-positive red cells. Exposure can happen in many ways:

• Delivery of an RhD-positive baby (vaginal or caesarean)
• Feto-maternal haemorrhage during pregnancy (antepartum haemorrhage, placenta praevia or abruption, abdominal trauma, external cephalic version)
• Miscarriage, ectopic pregnancy, molar pregnancy
• Pregnancy termination (surgical or medical)
• Invasive obstetric procedures (amniocentesis, chorionic villus sampling, fetal blood sampling)
• Stillbirth or intrauterine fetal death
• Earlier transfusion of RhD-positive blood components
• Intravenous drug use with shared equipment (rare)

Risk is raised when the mother's first ever exposure is not followed by anti-D, when the father or sperm donor is RhD positive (or of unknown status), and by any event that increases feto-maternal bleeding. A history of a previously affected baby is the strongest predictor of severe disease in the next pregnancy. ^[4][8]

Figure 1OSC Microbio 19 01 HDN

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Long description

OpenStax Microbiology figure 19.6 of haemolytic disease of the newborn: panel (a) shows an Rh-negative pregnant person exposed to Rh-positive fetal red cells, with anti-Rh IgG crossing the placenta in a later pregnancy and destroying fetal Rh-positive red cells, and panel (b) shows Rho(D) immune globulin binding fetal Rh-positive cells to prevent the primary immune response.

RhD is highly immunogenic. As little as 0.1 mL of RhD-positive red cells in the maternal circulation can trigger sensitisation. ^[9]

The sequence is:

1. First exposure (usually late in a pregnancy or at delivery) introduces RhD-positive fetal red cells into the mother's circulation.
2. The mother mounts a primary response dominated at first by IgM, which is too large to cross the placenta and does not harm the current fetus. B-cell memory is established. This is why the first RhD-incompatible pregnancy is usually unaffected.
3. On re-exposure in the next (or later) RhD-positive pregnancy, a rapid secondary response produces IgG anti-D, which does cross the placenta.
4. Maternal anti-D coats fetal RhD-positive red cells, which are cleared by macrophages in the fetal liver and spleen and by natural killer cells, producing haemolysis.
5. The fetus becomes anaemic; erythropoiesis expands in the bone marrow and liver/spleen (extramedullary), causing hepatosplenomegaly, and the breakdown of haemoglobin raises unconjugated bilirubin. Hydrops fetalis (fetal oedema, ascites, pleural and pericardial effusions) develops in severe cases.
6. Because IgG persists for years, the antibody concentration may be boosted by each pregnancy rather than fade.

Mother-neonatal consequences are mirror images: antenatal anti-D destroys fetal red cells, and postnatally the same antibodies persist in the baby's circulation for several months, causing late neonatal anaemia.

The mother is almost always asymptomatic. The condition is found on screening rather than by symptoms.

In an affected pregnancy, the fetus may show reduced movements, growth restriction, polyhydramnios (from hydrops), or frank hydrops on ultrasound. Severity ranges from mild compensated anaemia to stillbirth. ^[3][9]

In the neonate, look for:

• Jaundice appearing within the first 24 hours (unconjugated hyperbilirubinaemia)
• Pallor and anaemia, a positive direct antiglobulin (Coombs) test on cord blood
• Hepatosplenomegaly, and in severe disease oedema/ascites/hydrops
• Kernicterus (bilirubin encephalopathy) if hyperbilirubinaemia is severe and untreated

No specific maternal physical sign. Routine antenatal examination continues; the important "findings" are laboratory (blood group, antibody screen, antibody concentration) and sonographic (MCA Doppler, signs of hydrops).

A positive antibody screen or a jaundiced, anaemic newborn does not automatically mean RhD disease.

Investigations divide into three groups: maternal status, fetal status, and, at delivery, the volume of feto-maternal haemorrhage.

A clinically significant rise is an anti-D concentration increasing by more than 50%. Affected fetuses are usually delivered around 37 weeks to limit further exposure to maternal antibody. ^[8]

After any potentially sensitising event (PSE): offer anti-D immunoglobulin as soon as possible, and within 72 hours. Each new PSE needs its own dose, independent of routine antenatal prophylaxis. Do not wait for the Kleihauer result before giving the first dose, and, for postpartum prophylaxis, benefit may still be seen up to 28 days later. ^[9][1] Routine antenatal anti-D prophylaxis (RAADP): check RhD status and an antibody screen at booking; repeat at 28 weeks before the dose is given. ^{[4][6] [9] [10]}

Schedules used by major guidelines differ:

Weak and partial D: guidelines differ; for example some (such as BSH) treat weak D types 1-3 as RhD positive and avoid prophylaxis, and other variants are managed as RhD negative through the transfusion laboratory. ^[2] In practice, the local laboratory report should drive the decision, not a bedside label.

Critical early-pregnancy point: most guidelines, including WHO (2016), RCOG, ACOG in its December 2024 clinical practice update, SOGC, SFP and NAF, no longer advise routine RhD testing and anti-D immunoglobulin before 12 weeks' gestation, because evidence of benefit in early pregnancy loss is limited; ACOG still allows case-by-case use after shared decision making. Most experts give the full 300 microgram dose at or beyond 12 weeks. ^{[1][7] [9]} Postpartum dose: within 72 hours if the baby is RhD positive, after a D typing of cord blood (or maternal/cord sample if D type unknown). ^[8] A standard 300 microgram dose covers up to 15 mL of fetal red cells; larger bleeds need extra vials guided by Kleihauer-Betke or flow cytometry. ^[9] A woman who is confirmed to be alloimmunised (immune, not passive) should not receive anti-D (it no longer helps). ^{[6] [1]}

This pathway is for RhD-negative (or any antibody-positive) women in whom immune anti-D has been identified.

In general:

• Determine fetal RHD genotype from maternal plasma (from 16 weeks in alloimmunised pregnancies) so an RhD-negative fetus can be spared invasive surveillance. ^[8]
• Serial antibody monitoring detects a rising trend (more than a 50% increase is significant) and any additional antibodies. ^[8]
• MCA Doppler surveillance begins at 18-22 weeks, or earlier after a critical titre, and continues every 1-2 weeks; more than 1.5 multiples of the median predicts fetal anaemia with reasonable sensitivity and a false-positive rate of roughly 13-18%. ^{[9] [8]}
• Cordocentesis confirms fetal haemoglobin when Doppler is suspicious; a haemoglobin at least 2 standard deviations below the mean (around 0.84 multiples of the median) or a haematocrit of 30% is an indication for intrauterine transfusion (IUT), which is usually performed between 18 and 35 weeks using fresh, cross-matched (to maternal plasma), irradiated, leukocyte-depleted red cells. ^[9]
• Hydrops on ultrasound is itself an indication for cordocentesis and usually for transfusion. ^[9]
• Delivery is usually planned at about 37 weeks, or earlier if repeated intrauterine transfusions were needed or severe anaemia develops, with antigen-negative blood available for the neonate. ^[8][9]

Neonatal follow-up matters as much as the pregnancy: after delivery the baby may need phototherapy, exchange transfusion for severe hyperbilirubinaemia, or later "top-up" transfusion for ongoing haemolysis, and should be monitored for late hyporegenerative anaemia. ^[1]

• Fetal anaemia, which can progress to hydrops fetalis and stillbirth if untreated.
• Hyperbilirubinaemia and kernicterus in the newborn, with sensorineural deafness, choreoathetosis and cognitive impairment.
• Late neonatal anaemia (hyporegenerative or ongoing haemolysis) weeks after birth.
• Iron overload with multiple intrauterine transfusions.
• Cordocentesis and intrauterine transfusion complications: fetal bradycardia, bleeding, infection, preterm birth.
• Transfusion strategy pitfalls (for example using the wrong Rh phenotype of donor cells in anti-c or anti-e disease) need specialist blood bank input. ^[9]

• Where a prophylaxis programme exists, RhD alloimmunisation is now rare and perinatal outcomes for affected pregnancies are good, provided surveillance and transfusion are available. ^[2]
• Alloimmunisation recurrence: a woman who has made anti-D stays sensitised for life; subsequent RhD-positive pregnancies are at risk and usually need surveillance from early in pregnancy. ^[8]
• After a first intrauterine transfusion, survival and neurodevelopment are good, but pregnancies requiring repeated transfusions have higher complication rates. ^[9]
• Prognosis in countries without prophylaxis remains poor: stillbirth and neonatal death/brain injury in a large minority. ^[2]

• Every pregnant woman should have RhD typing and an antibody screen at booking and again at 28 weeks. ^[4][6]
• RhD-negative, unsensitised women should be offered anti-D after any sensitising event and routine antenatal anti-D prophylaxis under a defined schedule. ^[4]
• Fetal RHD genotyping from maternal blood allows targeted prophylaxis and reduces the number of women receiving unnecessary anti-D. ^[6][8]
• Use of Kleihauer/Rosette at delivery to decide whether the postpartum dose given (standard or extra) was enough. ^[8][9]
• Avoiding RhD-positive transfusion in women under 50 who are RhD negative or of unknown status (relevant for later pregnancies). ^[8]
• First-degree relatives and familial cases: think of inherited anaemia? Not applicable to RhD.

Pregnancy. This entire topic is pregnancy-focused. Additional points:

• Partner RhD status affects risk; where the father is known to be RhD negative, routine antenatal prophylaxis is unnecessary. Paternity testing should not be used to avoid surveillance in an already sensitised pregnancy. ^[9]
• Multiple gestation and pregnancy complicated by bleeding need individualised dosing and more frequent Kleihauer testing.
• Breastfeeding does not influence alloimmunisation.

Children. RhD alloimmunisation itself does not occur in children, but neonates with HDFN need bilirubin and haematological monitoring for days to weeks and sometimes "top-up" transfusions; late hyporegenerative anaemia can appear 2-6 weeks after birth. ^[1]

Older adults. Not directly relevant, although women with a remote history of alloimmunisation may need remembered sensitisation in any future (for example donor) context. RhD incompatibility does not cause disease in non-pregnant adults.

Low-resource settings. ACOG's abstract stresses that haemolytic disease remains a major cause of death and brain injury where prophylaxis programmes are absent. ^[2] WHO's 2016 guideline describes routine antenatal anti-D as a context-specific (research) recommendation, while affirming postnatal anti-D with high-certainty evidence. ^[7] Practical priorities where capacity is limited:

• Universal RhD typing and antibody screening at booking, even if nothing else is added. ^[4]
• Give postpartum anti-D within 72 hours to every RhD-negative woman delivered of an RhD-positive (or untyped) baby, because this single intervention has the strongest evidence and the largest effect. ^[7][1]
• Prioritise one 1500 IU dose inserted into routine antenatal care (at 28 weeks) where supply and cold chain permit. ^[9]
• Kleihauer/Rosette can be replaced, where unavailable, by a single standard dose for every delivery, accepting that very large haemorrhages will be under-treated.
• Start a surveillance service (MCA Doppler, IUT) only where fetal medicine expertise exists; otherwise refer complicated pregnancies early to the nearest centre that has it. ^[9]

See the differential diagnosis table above. In summary: RhD alloimmunisation is antibody-mediated, threatens the current and all future RhD-positive pregnancies, can be predicted by titres and by transplacental Doppler, and is treated antenatally with intrauterine transfusion as well as neonatally. ABO incompatibility is more common but usually milder, while passive anti-D needs to be recognised as not immune.

• RhD-negative women carrying an RhD-positive fetus can become sensitised by delivery, bleeding, miscarriage, abortion, ectopic pregnancy, trauma, ECV or prior transfusion of RhD-positive blood. ^[1]
• Sensitisation usually spares the first pregnancy; the dangerous illness appears in the next RhD-positive pregnancy, mediated by IgG anti-D crossing the placenta. ^[9]
• Typing at booking and at 28 weeks (before any anti-D is given), plus anti-D after sensitising events and routine antenatal prophylaxis, reduce alloimmunisation to well under 0.2% of at-risk pregnancies. ^[2]
• A standard 300 microgram (1500 IU) dose of anti-D protects against up to about 15 mL of fetal red cells (30 mL whole fetal blood); larger bleeds need extra doses guided by Kleihauer-Betke or flow cytometry. ^[9]
• Unsensitised women need prophylaxis; women with immune (not passive) anti-D must not receive anti-D. ^[6]
• Passive anti-D rarely exceeds 0.4 IU/mL after a single 1500 IU dose; immune levels are stable or rising and are interpreted against 4 IU/mL and 15 IU/mL thresholds. ^[8]
• A sensitised pregnancy is managed with monthly-then-fortnightly antibody quantification, fetal MCA Doppler from 18-22 weeks, and intrauterine transfusion when MCA PSV exceeds 1.5 multiples of the median. ^[8][9]
• Most affected pregnancies are delivered around 37 weeks. ^[8]
• Hydrops and kernicterus are the feared complications; both are now rare where screening and prophylaxis programmes run. ^[2]
• WHO's 2016 stance is that routine antenatal anti-D is context-specific (research) while postnatal anti-D rests on high-certainty evidence. ^[7]