Rh Factor: What’s Better, Positive or Negative Results?

Understand when and why it matters

Medically reviewed by Chioma Ndubisi, MD

Blood typing includes the identification of A and B sugar antigens and Rh proteins on the surface of the red blood cells (RBCs). The presence or absence of Rh proteins on red blood cells is used when describing a blood type as positive (+) or negative (-).

Your Rh type has no general effect on your health. It only becomes important during pregnancy or when you need a blood transfusion. Pregnant people who are Rh negative must take precautions against developing anti-Rh antibodies, which can affect the health of future pregnancies.

This article will describe what Rh factor is, when it matters, and what you should do if you find out that you are Rh negative or Rh positive, especially during pregnancy.

Rh Factor: Meaning of Blood Type in Pregnancy

Normally, blood testing is part of early prenatal care. Blood testing during the first trimester of pregnancy includes blood typing, RBC count, white blood cell (WBC) count, platelet count, and electrolyte levels.

The ABO blood type system is defined as type O (the absence of A or B antigens), type A, type B, and type AB. Each of these types is followed by the Rh type (positive or negative), such as A+ or O-. This is a shorthand for whether or not Rhesus (Rh) proteins are present on the surface of the RBCs. The Rh type is inherited separately from the ABO type.

Most people are Rh positive. According to the American Red Cross, about 10% of the U.S. population is Rh negative, and this percentage varies by race.

Rh incompatibility can complicate pregnancy for an Rh negative pregnant person who is carrying an Rh positive fetus. It does not affect pregnancy if the pregnant person is Rh positive, regardless of the Rh type of the fetus.

Blood from an Rh positive fetus can enter the bloodstream of the pregnant person and stimulate the production of anti-Rh antibodies. This is called Rh sensitization. In a future pregnancy, these antibodies can attack the blood cells of an Rh positive fetus.

What Are Antibodies?

Antibodies are immune proteins that form in response to antigen exposure. Antigens are proteins or other substances that are not part of your body’s own structure. Common antigens your body is exposed to include those found on viruses and bacteria.

If you are exposed to someone else’s blood (such as a blood transfusion or from a fetus during pregnancy), your body would recognize Rh proteins as antigens if your own RBCs don’t have these proteins. You would form antibodies to destroy RBCs that are Rh positive. It takes weeks to months for these antibodies to develop. These antibodies can cross the placenta in pregnancy and harm an Rh positive fetus.

Rh Factor of Each Parent

Having Rh factor protein on the surface of the RBCs is a dominant trait. This means that if one of a baby's genetic parents is Rh positive, the child may be Rh positive, even if the other parent is Rh negative.

Each person has two genes for Rh factor. If one or both of those genes code for being Rh positive, they are Rh positive. If both of their genes code for being Rh negative, they are Rh negative.

If both of their genes code for being Rh positive, all of their genetic offspring will be Rh positive, regardless of the Rh type of the other genetic parent.

If a person has one Rh positive gene and one Rh negative gene, they will test as being Rh positive. However, there is only a 50% chance for any of their offspring to inherit the Rh positive gene. If the other genetic parent is Rh negative, there is a 50% chance the baby will be Rh positive.

If a person is pregnant with a fetus conceived with their own egg and the sperm of the other genetic parent, Rh factor can affect the pregnancy in these ways:

• If a pregnant person is Rh negative and the other genetic parent is Rh positive, the fetus may be Rh positive. There may be Rh incompatibility.

• If a pregnant person is Rh negative, and the other genetic parent is Rh negative, the fetus will be Rh negative, and there is no risk of Rh incompatibility.

• If a pregnant person is Rh positive, there is no risk of Rh incompatibility, regardless of the other parent's Rh type.

If the pregnant person is carrying a fetus conceived with another person's egg through in vitro fertilization (such as in surrogacy or egg donation), Rh factor can affect the pregnancy in these ways:

• If the pregnant person is Rh negative and is carrying a fetus conceived by another person's egg with at least one Rh positive genetic parent, there may be Rh incompatibility.

• If the pregnant person is Rh positive, there is no risk of Rh incompatibility, regardless of the Rh status of the genetic parents (who contributed the egg and sperm).

Rh Factor Incompatibility, Antibodies, and Fetus Risks

Blood typing is important during pregnancy because Rh incompatibility can lead to serious problems for the fetus. Rh incompatibility means that the pregnant person is Rh negative, and the fetus is Rh positive.

This can cause an immune-driven hemolytic reaction in the blood of the fetus or baby. Hemolysis is the breaking down of RBCs. It causes anemia (low RBCs) and organ failure affecting the fetus or baby.

The survival of a hemolytic reaction due to Rh incompatibility is variable. Fetuses and babies who experience mild anemia can survive if treatment is initiated early, but many can develop lifelong disability or might not survive if the reaction is severe.

When Do Rh Antibodies Form?

A pregnant person who is Rh negative may produce antibodies (immune proteins) against the Rh proteins on the fetus's red blood cells. Most often, the fetus’s blood doesn’t mix with the pregnant person's blood until labor and delivery.

However, anti-Rh antibody formation can occur after amniocentesis and other invasive prenatal testing procedures. A person may also form Rh antibodies due to ectopic pregnancy, miscarriage, or pregnancy termination if there is Rh incompatibility.

Antibody formation after the first exposure to Rh factor takes time, so it usually doesn't affect the pregnancy in which the sensitization occurs. Also, Rh factor doesn't develop on the fetus's red blood cells before eight weeks of pregnancy, so early pregnancy loss or termination is less of a risk.

If you are Rh negative, you might need medication to prevent anti-Rh antibody formation during your pregnancy and/or after delivery. The medication, RhoGAM, or Rho(D) immune globulin, would be necessary for preventing the development antibodies if you are carrying a fetus that was conceived with your own egg cell or a fetus conceived by a donor egg cell.

Blood transfusions are given after blood is typed and crossmatched to ensure there is no Rh incompatibility. In an emergency, when there is no time for blood typing, type O Rh negative blood is given. Therefore, blood transfusion is a very rare reason a person would form Rh antibodies.

Incompatible Rh Factor Injection and Future Pregnancies

If you are Rh negative, the treatment you would get during your pregnancy is guided by whether antibodies are present in your blood.

Preventing Anti-Rh Antibody Formation

For Rh negative pregnant people who have not formed any Rh antibodies, RhoGAM will be given by injection at about 28 weeks of pregnancy to prevent the formation of these antibodies. Sometimes injections may be scheduled sooner if you are going to have an amniocentesis or have had a pregnancy loss or termination.

If the baby is found to be Rh positive at birth or is not tested for Rh factor, the Rh negative person will receive an additional dose of RhoGAM within 72 hours after delivery.

Use of RhoGam

If a person with Rh antibodies becomes pregnant with an Rh positive fetus, the already-formed antibodies can quickly attack the fetus’s blood, causing severe hemolysis. Therefore, prevention of antibody formation is used during a first pregnancy or at the time of a miscarriage or pregnancy termination.

When Rh Antibodies are Present

If you already have Rh antibodies during your pregnancy (usually due to a previous Rh incompatible pregnancy), your pregnancy would be considered high risk. Early delivery of the baby might be recommended to reduce the likelihood of hemolysis. Your baby’s blood levels would be monitored to detect whether they need a blood transfusion.

Transfusions and Rh Negative Blood

When donor blood is used for a blood transfusion, it is typed and matched for compatibility. This includes matching the Rh type and A, B, AB, and O blood types (the ABO system).

If a person is Rh negative and needs a blood transfusion, the donor blood used in a transfusion must be Rh negative and of a compatible ABO type.

If an Rh negative person receives an Rh positive blood transfusion, the effect depends on whether they have been exposed and sensitized before. If they don't have Rh antibodies already, the transfusion will not affect them immediately, but they may develop Rh antibodies.

Once a person has Rh antibodies, future transfusions of Rh positive blood may result in a hemolytic transfusion reaction in the person and can affect a pregnancy with an Rh positive fetus.

You can donate blood if you are Rh positive or Rh negative. Your blood would be typed and only given to recipients who would not be at risk of rejecting your blood during a blood transfusion.

The O negative blood type is considered a universal donor that would not be rejected recipients of any blood type. This type is used in emergency situations when there is not enough time to type the blood transfusion recipient for a match. The AB-positive blood type is considered a universal recipient who may receive blood of any type.

The ABO type must also be considered when giving a person a transfusion. If you are type A, you will have anti-B antibodies from early in life (without exposure to blood), and If you are type B, you will have anti-A antibodies. A type O person has both anti-A and anti-B antibodies, while a type AB person has neither of these.

If you are given a blood transfusion that is positive for the A and/or B proteins to which you have an antibody, a severe immune response will occur, with a potentially deadly hemolytic reaction.

Related: Why Do People Need Blood Transfusions?

Communicating Rh Factor Status During Pregnancy

If you are pregnant, you can expect to have Rh typing during the first trimester of your pregnancy. If you are Rh positive, no action needs to be taken. If you are Rh negative, you may need treatment to prevent your body from making Rh antibodies.

Antibodies that form in the first Rh incompatible pregnancy are not likely to cause a severe reaction during that pregnancy. Sensitization usually occurs during labor and delivery. Also, these first antibodies made are of the large IgM (immunoglobulin M) type that can't pass through the placenta to harm the fetus.

However, for subsequent pregnancies after being sensitized, the pregnant person's body is poised to make anti-Rh antibodies of the IgG (immunoglobulin G) type, which are smaller and can pass through the placenta to enter the fetus's bloodstream. During a subsequent pregnancy, the antibodies can attack an Rh positive fetus's RBCs.

Future Health of Babies in Rh Incompatible Pregnancies

If a person knows they are Rh negative before pregnancy or discover they are by testing during pregnancy, they can discuss any implications with a healthcare provider. In general, Rh negative fetuses and babies are not at risk of any harm by being Rh negative, regardless of the Rh type of the pregnant person.

The risk to Rh positive fetuses and babies carried by an Rh negative pregnant person is higher for those carried after the first pregnancy.

With the use of screening tests and the Rho(D) immune globulin shot, fewer Rh negative pregnant people develop anti-Rh antibodies that can affect future pregnancies. If they have developed an antibody, the health of the fetus will be closely monitored to look for signs of Rh incompatibility and the development of hemolytic disease of the fetus and newborn.

If the fetus is showing signs of mild anemia, they will be monitored and may not need any treatment before or after birth. For moderate anemia, early delivery may be recommended. After such a delivery, the baby may need a blood transfusion or treatment for jaundice (high bilirubin levels from the breakdown of RBCs causing yellowing of the skin and eyes).

For severe anemia, the fetus may receive a blood transfusion while still in the uterus. They may recommend a cesarean section (surgical delivery). After birth, the baby may need further transfusions and treatment for jaundice.

These steps can help prevent some of the severe consequences of hemolysis, such as hydrops (fluid throughout the body tissues), which can be fatal. Other effects can be an enlarged liver or kernicterus, in which there is brain damage due to high bilirubin.

Summary

Blood type is defined as positive or negative based on whether the Rh factor is present on the surface of the RBCs. Most people are Rh positive, but a significant minority are Rh negative.

Blood typing is important before a blood transfusion and as part of prenatal care. A person who is Rh negative should not receive Rh positive blood, but a person who is Rh positive can receive Rh positive or Rh negative blood.

Rh incompatibility occurs when an Rh negative pregnant person is carrying an Rh positive fetus. The fetal blood can mix with the pregnant person's and induce the formation of anti-Rh antibodies. This most often occurs during labor and delivery but may happen earlier in pregnancy, including lost or terminated pregnancies and during certain procedures. These antibodies can destroy fetal Rh positive RBCs in future pregnancies.

If there’s a risk of Rh incompatibility, a pregnant person can be treated with RhoGAM medication to prevent the formation of anti-Rh antibodies. If these antibodies have developed, a subsequent pregnancy with Rh incompatibility carries a high risk of severe health consequences for the fetus.