Rh Factor


What is Rh factor?

Rh factor, also called Rhesus factor, is a type of protein found on the outside of red blood cells. The protein is genetically inherited (passed down from your parents). If you have the protein, you are Rh-positive. If you did not inherit the protein, you are Rh-negative. The majority of people, about 85%, are Rh-positive.

Why is Rh factor important?

This protein does not affect your overall health, but it is important to know your Rh status if you are pregnant. Rh factor can cause complications during pregnancy if you are Rh-negative and your child is Rh-positive.

What is Rh incompatibility?

Rh incompatibility occurs when a woman who is Rh-negative becomes pregnant with a baby with Rh-positive blood. With Rh incompatibility, the woman’s immune system reacts and creates Rh antibodies. These antibodies help drive an immune system attack against the baby, which the mother’s body views as a foreign object.

Antibody formation can happen after blood transfusions or when fetal blood enters the mother’s circulation:

Who is at risk for Rh incompatibility?

A woman who is Rh-negative is at risk for Rh incompatibility when she becomes pregnant. Rh incompatibility happens only when the father of the baby is Rh-positive. Doctors do not routinely test men’s Rh status. Instead, expectant parents discuss their individual status with their doctor.

Symptoms and Causes

What causes Rh incompatibility?

A difference in blood type between a pregnant woman and her child causes Rh incompatibility. Children may be Rh-positive if they inherit the protein from their father, even if their mother is Rh-negative.

Diagnosis and Tests

How is Rh incompatibility diagnosed?

For most women, simple blood tests diagnose Rh incompatibility. If you become pregnant, your obstetrician will test you to determine if you are Rh-negative. Doctors usually do this test with your routine first trimester blood work. It may be done earlier if you have vaginal bleeding.

  • If you have Rh-negative blood, your doctor may order another blood test, called an antibody screen. This test checks whether your blood contains Rh antibodies. If your antibody screen comes back positive, you are at risk for Rh incompatibility.
  • If you are Rh-negative and your antibody screen is negative, you will be given Rh immunoglobulin (RhIg) to prevent antibody formation. This is typically given around 28 weeks and within 72 hours of delivery. You may get a dose in early pregnancy if you have bleeding or other complications.

Management and Treatment

What complications are associated with Rh incompatibility?

Rh incompatibility does not affect pregnant women. In a baby, it can cause hemolytic anemia. Hemolytic anemia causes a baby’s red blood cells to be destroyed faster than they can be replaced.

The effects of hemolytic anemia can range from mild to severe. These effects may include jaundice, liver failure, and heart failure. Doctors treat this condition quickly depending on its severity.

  • For mild cases, no treatment may be necessary.
  • For severe cases, a baby may receive a blood transfusion through the umbilical cord. This procedure helps replace the baby’s red blood cells.
  • Babies who have jaundice, or a large amount of bilirubin in the blood, may be treated with special lights to help reduce bilirubin levels.


Can Rh incompatibility be prevented?

Because Rh factor is genetic, it is not possible to choose which Rh type your baby has. However, if you are an Rh-negative woman with an Rh-positive baby, you can prevent Rh incompatibility by receiving RhIg at specific times during your pregnancy. It is an important topic to discuss with your healthcare provider.

Outlook / Prognosis

What is the outlook for women with Rh-negative blood?

While an Rh-negative woman will not be harmed by contact with Rh-positive blood, she will need RhIg injections after every contact with Rh-positive blood to reduce risks for babies in a future pregnancy. These events include:

  • Pregnancy, including miscarriage and abortion
  • Blood transfusions
  • Transplants involving blood or marrow cells
  • Accidental needle-sticks with Rh-positive blood

Last reviewed by a Cleveland Clinic medical professional on 12/03/2018.


  • National Heart, Lung, and Blood Institute. Rh incompatibility. (https://www.nhlbi.nih.gov/health/health-topics/topics/rh) Accessed 12/4/2018.
  • American College of Obstetricians and Gynecologists. The Rh factor: How it can affect your pregnancy. (https://www.acog.org/Patients/FAQs/The-Rh-Factor-How-It-Can-Affect-Your-Pregnancy) Accessed 12/4/2018.
  • American Pregnancy Association. Rh Factor. (http://americanpregnancy.org/pregnancy-complications/rh-factor/) Accessed 12/4/2018.

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