Rh incompatibility: What it is and who is at risk?

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Rh incompatibility is a medical condition that can occur during pregnancy when a pregnant person and their fetus have different Rh (Rhesus) blood types.

While it is preventable and manageable with appropriate prenatal care, failure to detect or treat Rh incompatibility in time can result in serious complications for the fetus, including hemolytic disease of the fetus and newborn (HDFN).

What Is the Rh Factor?

The Rh factor is a protein found on the surface of red blood cells. If the protein is present, a person is classified as Rh-positive; if it is absent, the person is Rh-negative.

Remember that these designations are separate from blood type classifications like A, B, AB, or O.

In most populations, the majority of individuals are Rh-positive. The condition known as Rh incompatibility occurs when an Rh-negative pregnant person is carrying an Rh-positive fetus. This situation can lead to an immune reaction if fetal red blood cells enter the maternal bloodstream.

How Rh Incompatibility Develops

Small amounts of the fetus’s blood may enter the maternal circulation during pregnancy or delivery. If the mother is Rh-negative and the fetus is Rh-positive, the maternal immune system may recognize the fetal Rh-positive cells as foreign. In response, it begins producing antibodies against them.

This immune process, called sensitization, usually does not affect the current pregnancy. However, in subsequent pregnancies with another Rh-positive fetus, the previously formed antibodies can cross the placenta and attack the fetus’s red blood cells.

This can result in HDFN, a potentially life-threatening condition marked by severe anemia, jaundice, heart failure, or hydrops fetalis.

Who Is at Risk?

Rh incompatibility only occurs if the pregnant person is Rh-negative and the fetus is Rh- positive.

If both parents are Rh-negative, there is no risk. If the father is Rh-positive, there is approximately a 50% chance the fetus will also be Rh-positive, placing the pregnancy at risk for incompatibility.

First pregnancies are generally unaffected unless the mother was previously sensitized through blood transfusions, miscarriage, abortion, or invasive prenatal procedures.

Once sensitization occurs, the maternal immune system remains primed to produce antibodies in future Rh-incompatible pregnancies.

Prevention and Management

Rh incompatibility can be prevented through routine prenatal screening. Blood tests early in pregnancy determine the Rh status of the pregnant person. If Rh-negative, the next step is testing the biological father’s Rh status or monitoring fetal Rh type if non-invasive testing is available.

To prevent sensitization, Rh immunoglobulin (RhIg)—commonly known as Rho(D) immune globulin—is administered. This injection contains antibodies that neutralize any fetal Rh- positive cells before the mother’s immune system can recognize and react to them.

RhIg is typically given around the 28th week of pregnancy and again within 72 hours of delivery if the baby is Rh-positive. It is also given after events like miscarriage, trauma, or amniocentesis.

In cases where sensitization has already occurred, the pregnancy is considered high-risk.

Maternal antibody levels are monitored, and fetal health is assessed using Doppler ultrasound and, in some cases, amniocentesis.

Conclusion

Rh incompatibility is a serious but preventable condition. Early screening, accurate diagnosis, and timely administration of Rh immunoglobulin have significantly reduced complications related to this condition.

With appropriate management, most individuals with Rh-negative blood can have safe pregnancies and healthy babies.