Rh incompatibility is a mismatch between a pregnant person’s blood type and their baby’s blood type that can trigger an immune reaction against the baby’s red blood cells. It happens when the mother is Rh-negative and the baby is Rh-positive, a trait inherited from the father. With modern prevention, serious complications are rare, but without treatment, subsequent pregnancies can be affected by progressively worsening attacks on the baby’s blood cells.
How Rh Factor Works
Your red blood cells carry surface proteins that define your blood type. One of these is the Rh D antigen. If you have it, you’re Rh-positive. If you don’t, you’re Rh-negative. About 15% of people in the United States and 17% in Britain are Rh-negative, though the rate varies dramatically by ethnicity. In China, Japan, and Indonesia, fewer than 1% of the population is Rh-negative. Among Basque populations in Europe and certain groups in Morocco, the rate reaches roughly 29%.
Rh factor is inherited in an autosomal dominant pattern, meaning a baby only needs one copy of the gene (from either parent) to be Rh-positive. So an Rh-negative mother and an Rh-positive father can easily produce an Rh-positive baby, setting the stage for incompatibility.
What Happens During Sensitization
During pregnancy and especially during delivery, small amounts of the baby’s blood can cross into the mother’s bloodstream. If the baby’s red blood cells carry the Rh D antigen and the mother’s don’t, her immune system recognizes those cells as foreign. On first exposure, the body produces large antibodies (IgM) that can’t cross the placenta, so the first baby is almost always unaffected.
The problem comes with the next Rh-positive pregnancy. The mother’s immune system now has a memory of the Rh D antigen, and it launches a rapid, aggressive response, producing smaller antibodies (IgG) that do cross the placenta. These antibodies attach to the baby’s red blood cells and destroy them. Once this sensitization process begins, it’s irreversible, and each subsequent Rh-positive pregnancy triggers a stronger antibody response.
Events That Can Cause Sensitization
Delivery is the most common trigger, but any event that allows fetal blood to mix with maternal blood can sensitize an Rh-negative mother. The American College of Obstetricians and Gynecologists identifies several specific situations:
- Miscarriage or abortion at 12 weeks or more of pregnancy
- Ectopic pregnancy
- Amniocentesis, chorionic villus sampling, or other procedures that access fetal tissue
- Bleeding after 20 weeks of pregnancy
- Abdominal trauma during pregnancy
- Manual attempts to turn a breech baby
Even early pregnancy losses before 12 weeks carry some risk, though it’s lower. Small amounts of fetal blood can enter the mother’s circulation without any obvious event, which is why preventive treatment is given routinely in the third trimester.
How It Affects the Baby
When maternal antibodies cross the placenta and attack fetal red blood cells, the result is hemolytic disease of the fetus and newborn. The severity ranges from mild to life-threatening, depending on how many antibodies the mother produces.
In mild cases, the baby develops jaundice after birth as destroyed red blood cells release bilirubin faster than the newborn liver can process it. Bilirubin levels that rise faster than 0.3 mg/dL per hour in the first 24 hours after delivery signal active hemolysis. Phototherapy (special light treatment) breaks down excess bilirubin in the skin.
In moderate cases, the baby becomes significantly anemic. More severe anemia requires exchange transfusion, where a portion of the baby’s blood is gradually replaced with compatible donor blood to remove both the harmful antibodies and excess bilirubin.
In the most severe cases, the baby develops hydrops fetalis while still in the womb. This occurs when fetal hemoglobin drops to roughly 7 g/dL below the normal level for gestational age, causing fluid to accumulate throughout the body. On ultrasound, this appears as abnormal fluid collections in two or more body compartments: swelling of the scalp tissue beyond 5 mm, fluid around the lungs or heart, or fluid in the abdomen. Ascites can appear as early as 20 weeks of pregnancy.
If bilirubin levels rise high enough in a newborn, unbound bilirubin can cross into the brain and cause kernicterus, a form of permanent brain damage. This is the complication that makes aggressive monitoring and treatment so important.
Screening and Monitoring
Every pregnant person gets a blood type and Rh factor test at the first prenatal visit. If you’re Rh-negative, your blood is also screened for anti-Rh antibodies using an indirect antiglobulin test, which checks whether your serum contains antibodies that could attack Rh-positive red blood cells. This screening is typically repeated at 28 weeks.
If antibodies are detected, the pregnancy is monitored more closely. The key tool is a specialized Doppler ultrasound that measures blood flow velocity in an artery in the baby’s brain. When a baby becomes anemic, the blood flows faster to compensate. A measurement above 1.5 times the expected median for gestational age is highly predictive of moderate to severe anemia, with a sensitivity of 100% and a false-positive rate of only 12%. This technique avoids the need for invasive fetal blood sampling in up to 70% of cases, reserving needle-based procedures for babies who truly need them. If the anemia is confirmed as severe (hematocrit below 30%), the baby may receive an intrauterine blood transfusion before birth.
Prevention With RhIg
The cornerstone of prevention is Rh immune globulin (commonly known by the brand name RhoGAM), an injection of pre-made antibodies that find and destroy any Rh-positive fetal red blood cells in the mother’s bloodstream before her own immune system can react to them. A standard dose is 300 micrograms, which protects against up to 30 mL of fetal blood entering the maternal circulation.
The standard prevention schedule has two doses: one at 26 to 28 weeks of pregnancy, and another within 72 hours after delivering an Rh-positive baby. The timing after delivery matters because the greatest transfer of fetal blood into the mother’s circulation happens during birth. If more than 12 weeks pass after the 28-week dose without delivery, a second antepartum dose is recommended.
Additional doses are given after any sensitizing event listed above. For pregnancies beyond 13 weeks, the standard 300 microgram dose applies. For losses before 12 weeks, the need for RhIg is evaluated on a case-by-case basis.
Determining the Right Dose After Delivery
In most deliveries, the standard 300 microgram dose of RhIg is sufficient. But occasionally, a larger-than-normal volume of fetal blood enters the mother’s circulation, such as during a traumatic delivery, placental abruption, or cesarean section. A blood test called the Kleihauer-Betke test measures the percentage of fetal red blood cells circulating in the mother’s blood. It doesn’t detect whether fetal blood mixing occurred; rather, it quantifies how much, so doctors can calculate whether additional doses are needed.
The math is straightforward: the percentage of fetal cells is multiplied by 50 to estimate the volume of fetal blood in milliliters, then divided by 30 (since each vial covers 30 mL). An extra vial is always added as a safety margin. Accurate dosing matters because too little RhIg leaves the mother at risk of sensitization, while too much means unnecessary treatment.
What Rh Incompatibility Means for Future Pregnancies
If prevention works and you never develop antibodies, each subsequent pregnancy follows the same protocol: screening, RhIg at 28 weeks, and another dose after delivery if the baby is Rh-positive. You can have multiple pregnancies without complications as long as sensitization is prevented each time.
If you do become sensitized, the situation is different. Antibody production is permanent, and RhIg no longer helps because it can only prevent the initial immune response, not reverse one that’s already established. Every future Rh-positive pregnancy will require close monitoring with Doppler ultrasound and potentially intrauterine transfusions or early delivery depending on severity. The antibody response tends to intensify with each successive pregnancy, meaning the risk to each subsequent Rh-positive baby increases.