Rhesus Disease: How Rh Incompatibility Affects Pregnancy

Rhesus disease occurs when a pregnant person’s immune system attacks their baby’s red blood cells because the two have incompatible Rh blood types. Specifically, an Rh-negative mother who has been exposed to Rh-positive blood can develop antibodies that cross the placenta and destroy fetal red cells, causing anemia that ranges from mild to fatal. In high-income countries, routine prevention with an injection called anti-D immunoglobulin has made severe cases rare, but the biology behind rhesus disease is more layered than a single shot can suggest, and globally the condition still harms more than 150,000 children every year.

How a Mother Becomes Sensitized

The Rh blood group system is controlled by two genes, RHD and RHCE, which encode proteins sitting on the surface of red blood cells. The D antigen is the one that matters most clinically. People who carry at least one working copy of the RHD gene are Rh-positive; those who lack functional copies are Rh-negative.¹ When an Rh-negative mother carries an Rh-positive fetus, small amounts of fetal blood routinely leak into her circulation during pregnancy and especially during delivery.² Her immune system recognizes the D antigen as foreign and begins producing anti-D antibodies.

This process, called sensitization or alloimmunization, usually does not cause trouble in the first pregnancy because the initial immune response is slow and the antibodies produced are mostly a type (IgM) too large to cross the placenta efficiently. The danger arrives in a subsequent pregnancy with another Rh-positive baby. Now the mother’s immune system mounts a rapid, secondary response with smaller IgG antibodies that pass freely into fetal blood and coat the baby’s red cells, marking them for destruction.

Delivery is the most common sensitizing event, but it is not the only one. Miscarriage, ectopic pregnancy, amniocentesis, abdominal trauma, and even routine prenatal procedures that disturb the placenta can introduce fetal red cells into the mother’s bloodstream. Blood transfusions with Rh-positive blood, though rare in modern practice, can also trigger sensitization. Any of these exposures can prime the immune system for an attack in a later pregnancy.

What Happens to the Fetus

Once maternal anti-D antibodies cross the placenta and bind to the D antigen on fetal red blood cells, those cells are flagged for removal by the fetal spleen and liver. The fetus tries to compensate by ramping up red cell production, but if destruction outpaces production, anemia sets in. Mild anemia may cause few problems. Moderate to severe anemia forces the fetal heart to pump harder, and the liver and spleen enlarge as they work overtime making new blood cells.

In the most severe cases, the fetus develops a condition called hydrops fetalis, where fluid accumulates in body cavities and tissues because the heart can no longer keep up with demand. Without treatment, hydrops is often fatal. Babies who survive severe anemia face another threat after birth: the rapid breakdown of antibody-coated red cells releases bilirubin, a yellow pigment the newborn liver is too immature to process efficiently. High bilirubin levels can cause jaundice and, if untreated, a form of brain damage called kernicterus.

Preventing Sensitization with Anti-D Immunoglobulin

The cornerstone of prevention is anti-D immunoglobulin, a blood product given to Rh-negative mothers to intercept any fetal red cells that have entered their bloodstream before the immune system has time to react. The injection works by coating fetal Rh-positive cells so they are cleared quickly without triggering a lasting immune memory. Most guidelines recommend a dose around 28 weeks of pregnancy and another within 72 hours of delivery, plus additional doses after any potentially sensitizing event such as a miscarriage or invasive procedure.

This strategy has been spectacularly successful in countries that have adopted it widely. Before its introduction in the late 1960s, rhesus disease was one of the leading causes of perinatal death. In high-income countries today, new sensitizations are uncommon, though not zero. Failures tend to occur when doses are missed, when the fetal bleed into the mother’s circulation is unusually large, or when a sensitizing event happens before the first routine injection.

Cell-Free DNA Testing and Targeted Prevention

A significant recent advance is noninvasive fetal RhD genotyping from a simple maternal blood draw. Fragments of fetal DNA circulate in the mother’s blood from early pregnancy onward, and laboratory analysis can determine whether the fetus carries the RHD gene. If the fetus is Rh-negative, there is no risk of rhesus disease and the mother does not need anti-D immunoglobulin at all.

The accuracy of these tests is striking. A U.S. study of over 400 Rh-negative pregnant individuals found that a cell-free DNA test correctly identified every Rh-positive fetus and every Rh-negative fetus, achieving 100% sensitivity and 100% specificity.³ A second large validation study of over 650 pregnancies confirmed near-perfect results, with 100% sensitivity for detecting Rh-positive fetuses and specificity above 99%.⁴ Even in the first trimester, when fetal DNA levels in maternal blood are lower, sensitivity has reached 100% in a study of over 400 samples, though specificity was somewhat lower at about 95%, with around 2% of results inconclusive.⁵

The practical payoff is substantial. In the UK, roughly a third of Rh-negative pregnant women carry Rh-negative fetuses and therefore receive anti-D injections they do not need.⁶ Targeted testing can spare those women unnecessary treatment and conserve a blood product that is derived from human plasma donors and is in limited supply globally. Several European countries, including Denmark and the Netherlands, have already incorporated routine fetal RhD genotyping into their national prenatal care programs. In the United States, adoption has been slower but is growing as commercial tests become more widely available.⁷

Monitoring an Already-Sensitized Pregnancy

When sensitization has already occurred and the fetus is Rh-positive, the clinical challenge shifts from prevention to surveillance. The goal is to detect fetal anemia early enough to intervene before the baby deteriorates. For decades this required amniocentesis, an invasive procedure that samples amniotic fluid and carries its own risks. A breakthrough came when researchers showed that Doppler ultrasound of the fetal middle cerebral artery could reliably detect anemia without a needle.

The principle is straightforward: an anemic fetus has thinner, less viscous blood, which flows faster. Measuring the peak speed of blood flow in the middle cerebral artery gives a noninvasive estimate of how anemic the fetus is. A landmark study in the New England Journal of Medicine demonstrated that this technique could detect moderate and severe anemia in fetuses at risk from maternal antibodies.⁸ Subsequent research confirmed that using a threshold of 1.5 standard deviations above the mean for gestational age catches about 96% of severely anemic fetuses, with a false-positive rate of roughly 14%.⁹ This technique has essentially replaced amniocentesis as the first-line monitoring tool for alloimmunized pregnancies.

Intrauterine Transfusion

When monitoring reveals that the fetus is becoming dangerously anemic, the treatment is intrauterine transfusion: a specialist inserts a needle through the mother’s abdomen, guided by ultrasound, and delivers compatible red blood cells directly into the fetal circulation, usually through the umbilical cord vein. The transfused cells are Rh-negative, so the mother’s antibodies cannot attack them, buying the fetus time until it is mature enough for delivery.

The procedure requires specialized expertise, and outcomes depend heavily on the center performing it. A 14-year series from a European referral center reported that intrauterine transfusion for red cell alloimmunization is safe when done by experienced teams, with the best outcomes seen when patients are referred before hydrops develops and when the needle is placed into the intrahepatic portion of the umbilical vein rather than a free-floating loop of cord.¹⁰ In severely affected pregnancies where transfusion begins before 22 weeks, survival rates of about 80% have been reported even when the fetus is profoundly anemic, sometimes requiring up to nine transfusions over the course of the pregnancy.¹¹

After the Baby Is Born

Delivery does not end the disease. Maternal antibodies persist in the newborn’s blood for weeks, continuing to destroy red cells. In the early days after birth, the main threat is hyperbilirubinemia. Phototherapy, which uses blue-spectrum light to break down bilirubin in the skin, is the first line of treatment. In severe cases, an exchange transfusion, where the baby’s blood is gradually replaced with compatible donor blood, may be needed to bring bilirubin down and wash out circulating antibodies.¹²

A second, often overlooked phase follows. Over the weeks after birth, as the baby’s own bone marrow ramps up red cell production and the remaining maternal antibodies gradually clear, a period of late anemia can develop. Babies who received intrauterine transfusions are at particular risk because their own red cell production may have been suppressed by the repeated transfusions. Monitoring hemoglobin levels through the first couple of months of life is standard practice for affected infants, and some will need additional transfusions after discharge.

The Curious Protection of ABO Incompatibility

An interesting quirk of blood group biology is that ABO incompatibility between mother and fetus actually reduces the risk of Rh sensitization. When the mother’s blood type is O and the fetus is type A or B, any fetal red cells that enter the mother’s circulation are quickly destroyed by her pre-existing anti-A or anti-B antibodies before her immune system has a chance to notice the D antigen and mount a response against it.

The effect is dramatic. A large statistical analysis of over 1,700 Rh-negative women found that the rate of Rh sensitization was about 15% in ABO-compatible pregnancies but only about 2% in ABO-incompatible ones.¹³ This natural protection was actually one of the clues that led researchers to develop anti-D immunoglobulin in the first place: if clearing fetal cells quickly prevented sensitization, then an injected antibody that did the same thing should work too.

Beyond Rh(D): Kell and Other Antigens

Rhesus disease usually refers to anti-D alloimmunization, but the same principle applies to dozens of other red cell antigens. After anti-D, antibodies against the Kell antigen (anti-K) are among the most clinically significant, and they cause fetal anemia through a different mechanism. While anti-D antibodies destroy circulating red blood cells, anti-Kell antibodies go a step further: they attack the progenitor cells in the fetal bone marrow that are still developing into red blood cells, suppressing production at its source.¹⁴ This means Kell disease can cause severe anemia with relatively less jaundice than classic Rh disease, because fewer mature red cells are being broken down.¹⁵

Other Rh antigens within the same blood group system can also cause problems. Antibodies against the c and E antigens, produced by the RHCE gene, have been reported to cause significant neonatal jaundice and anemia.¹⁶ Antibodies to antigens in the Duffy, Kidd, and MNS blood group systems have been documented as well, though severe cases are considerably rarer.¹⁷ Importantly, there is no equivalent of anti-D immunoglobulin for these other antigens. The only prevention strategy is careful antibody screening during pregnancy so that affected pregnancies can be identified and monitored early.

Weak D and Partial D Variants

Not everyone’s Rh status falls neatly into positive or negative. Some people carry variant forms of the RHD gene that produce a D antigen in reduced quantity (weak D) or with an altered shape (partial D). Standard blood typing can struggle with these variants, sometimes calling the same person Rh-positive in one lab and Rh-negative in another.¹⁸

The distinction matters clinically. Most people with the common weak D types (types 1, 2, and 3 in populations of European descent) produce enough D antigen that they are not at risk of forming anti-D antibodies, meaning they can safely be treated as Rh-positive and do not need anti-D immunoglobulin during pregnancy. People with partial D, on the other hand, are missing parts of the D protein and can become sensitized if exposed to complete D antigen. A case report illustrates the confusion these variants can cause: a patient was typed Rh-positive at one facility and Rh-negative at another, leading to a complicated series of treatments that could have been avoided with genetic testing from the start.¹⁹ Molecular RHD genotyping can resolve the ambiguity definitively but is not yet routine in most settings.

The Global Prevention Gap

While rhesus disease has become rare in wealthy countries, it remains a major cause of newborn illness and death in much of the world. A global analysis estimated that the disease affects more than 150,000 children annually in low- and middle-income countries, causing thousands of stillbirths and neonatal deaths along with cases of kernicterus and lasting brain damage.²⁰

The bottleneck is supply. Manufacturing anti-D immunoglobulin requires plasma from donors who already have anti-D antibodies, either from prior sensitization or from deliberate immunization programs. These programs are resource-intensive and ethically complex, and the global supply falls far short of demand. Researchers have estimated that roughly 13 million annual doses would be needed worldwide to provide both prenatal and postnatal prophylaxis to all Rh-negative pregnant women, yet fewer than 4 million doses are actually administered each year. Even covering postnatal-only prevention would require over 5 million doses, a threshold the world currently does not meet. Outside high-income countries, an additional 2.5 million doses per year are needed just to achieve basic postpartum coverage.²¹

The problem is not just manufacturing capacity. In many settings, Rh typing is not routinely performed during prenatal care, sensitization is not recognized until a baby is born sick, and the cold-chain infrastructure needed to store anti-D immunoglobulin may not exist. Solving the global rhesus disease burden will require coordinated efforts across healthcare systems, manufacturers, and governments, not just more vials of immunoglobulin.

Recombinant Anti-D and the Future of Prevention

One potential solution to the supply problem is recombinant anti-D immunoglobulin, a lab-manufactured version that would not depend on human plasma donors. A randomized trial compared a recombinant anti-D product to conventional plasma-derived anti-D in Rh-negative women who had delivered Rh-positive babies. By six months after injection, the two products performed similarly: the rate of positive antibody screening tests did not differ significantly between groups.²² No recombinant product has yet reached widespread clinical use, but the concept is being pursued because it could, in principle, be manufactured at scale without relying on a limited pool of immunized donors.

Why Rh-Negative Blood Still Exists

Given that rhesus disease kills babies, you might expect natural selection to have eliminated the Rh-negative trait long ago. The fact that roughly 15% of people of European descent are Rh-negative has puzzled geneticists for decades. One intuitive hypothesis is that being Rh-negative confers some hidden survival advantage, similar to the way the sickle cell trait protects against malaria. But genetic analyses of the RHD deletion have found no clear evidence that positive natural selection drove the trait to its current frequency.²³

A more mundane explanation may suffice. The initial rise in Rh-negative frequency could have been a matter of chance, a genetic drift event or founder effect in ancestral European populations. And once the Rh-negative trait reached an intermediate frequency, the selective pressure against it from rhesus disease turns out to be surprisingly weak. The disease only affects Rh-negative mothers carrying Rh-positive fetuses, and even then only after a sensitizing event, and even then is partly blunted by ABO incompatibility. The evolutionary math works out such that, near a frequency of about 50% for the RHD deletion, selection is too feeble to push it in either direction. Drift alone could maintain it more or less indefinitely. The Rh-negative trait may persist not because it helps anyone, but because it does not hurt quite enough for evolution to remove it.