Rh Factor in Pregnancy

For roughly 10 percent of pregnant women, knowing your Rh factor can be a matter of life or death for your baby or for your future unborn babies!

Ninety percent of people have Rh-positive blood, and therefore have no need to worry about Rh factor disease.

But women who have Rh-negative blood must be managed properly during pregnancy and postpartum to prevent potentially devastating future consequences.

The Rh (Rhesus) factor is a genetic trait that determines whether a certain protein is found on the surface of your red blood cells.

If you have this protein, you are Rh positive. If you do not have it, you are Rh negative. Whether you are Rh positive or negative doesn’t matter in the ordinary course of your life, as it does not affect your health.

It can affect your pregnancy, however, if your Rh factor is negative and your unborn baby is Rh positive. This condition is known as Rh incompatibility.

The baby’s blood type is inherited from the mother and father. An Rh-negative parent can only pass on an Rh-negative blood type to his or her offspring.

An Rh-positive parent, however, can pass on either an Rh positive or an Rh-negative blood type, depending on that person’s DNA.

Thus, if both parents of the baby are Rh negative, the baby will always be Rh negative. If the father is Rh positive and the mother is Rh negative, the baby usually will be born Rh positive, resulting in Rh-incompatibility.

This condition has the potential to create Rh sensitization in the mother, a very undesirable development (see below).

In most cases complications do not arise since this condition is usually diagnosed early in pregnancy and treated with a blood product called Rh immunoglobulin (a well-known version of this is called RhoGAM).

This is given as a one-time injection. Rh immuno-globulin prevents your immune system from making antibodies that can attack your baby’s blood during pregnancy.


During pregnancy (rarely), but typically during delivery, there is a chance that some of your baby’s red blood cells will mix with your blood.

If you are Rh negative and your baby is Rh positive, your body may respond by producing antibodies that attack the Rh-positive red blood cells.

This process is called Rh sensitization.

In most cases this isn’t a problem if it’s your first pregnancy, since the number of antibodies produced initially isn’t enough to harm the baby, and because the sensitization occurs after the baby has delivered.

If you do not receive the Rh immunoglobulin treatment, however, your body may continue producing antibodies against Rh-positive blood for the rest of your life. (Interestingly, this only happens in about 15 percent of pregnancies thus exposed).

If you become pregnant a second time and the fetus has Rh-positive blood again, these antibodies can cross the placenta and attack the fetal red blood cells.

This leads to a steady loss of blood in the fetus as the fetal red blood cells attacked by these Rh antibodies are removed from the fetal circulation. This can cause fetal anemia, and possibly lead to a severe condition called fetal hydrops, or hydrops fetalis.

Hydrops fetalis is a potentially fatal consequence of Rh incompatibility. The severe fetal anemia causes the heart to work extremely hard to circulate fetal blood. Eventually the heart swells and goes into heart failure.

This causes severe edema (swelling) in the baby’s entire body, creating what is called a hydropic fetus.

There are other conditions that can cause fetal hydrops, such as fetal heart defects or viral infections of the fetus during pregnancy (such as Parvovirus B19). Those are referred to as non-immune hydrops.

If fetal anemia is detected during pregnancy, specialists have the ability to perform a fetal blood transfusion during the pregnancy. Fortunately, due to the widespread use of Rh immunoglobulin, this is rarely necessary.

Common findings associated with Rh incompatibility include:

  • Excessive amniotic fluid surrounding the unborn infant (diagnosed by fetal ultrasound)
  • Poor muscle tone in the infant after it is born
  • Delayed motor and mental development
  • Prolonged jaundice

The Rh Factor Blood Test

If you are pregnant, your healthcare provider is required BY LAW to test your blood type and Rh factor early in the pregnancy.

Every woman should know her blood type! This basic blood test determines whether you are Rh positive or negative. There are no risks involved with this test, nor do you need any special preparation.

If you are Rh negative, then commonly the father of the baby is also tested. If you and the father both test Rh negative, your baby will be Rh negative and there is no cause for concern.

Likewise, if you and the father both test Rh positive, the baby will also be Rh positive. Another routine pregnancy blood test called an antibody screen is performed in order to make sure that you are not already Rh-sensitized.

Some OB doctors are so risk-averse that they will provide Rh antibody treatment to an Rh-negative pregnant woman regardless of the father’s blood type.

One rationale for this is the remote chance that the lab made an error on the father’s blood type. Another is that occasionally the woman’s “partner” is not the actual father of the baby!

There is a controversial new blood test for Rh-negative pregnant women called the fetal D antigen test. During the pregnancy, the mother’s blood can be tested for the presence or absence of fetal Rh DNA, known as the D antigen DNA.

If this test shows no D antigen DNA present in the mother’s blood, the assumption is that the baby must be Rh negative and therefore the pregnant woman does not need any Rh immunoglobulin.

Relying on this result and failing to administer Rh immunoglobulin could be thought of as “betting your baby’s life” on the results of this one test, however, and many OB doctors are unwilling to do that.

If you are Rh negative, you will need an Rh immunoglobulin injection about the 28th week of pregnancy (earlier for twins).

The antibody screen will be repeated before administering the Rh immunoglobulin medicine to confirm that you have not become Rh sensitized.

After you receive this medicine, the antibody screen can remain positive for the rest of your pregnancy because it will detect the medicine that you received.

Rh Immunoglobulin

The Rh immunoglobulin injection, also known as RhIG (RhoGAM is the most well-known but there are others), is typically given around the 28th week of pregnancy and usually again after delivery, within 72 hours.

After your baby is born, his or her blood type is tested. If the baby is Rh positive, you get the shot. If the baby is Rh negative, no shot!

Not only does the immunoglobulin prevent the mother’s immune system from producing antibodies to Rh-positive blood, it also destroys any red blood cells from the fetus that may have entered her system before her body began developing the antibodies.

Being Rh Negative

During pregnancy, any time your baby’s blood can potentially come into contact with your Rh-negative blood, you’ll need to take steps to ensure your safety. For any of the following situations, you may need an extra treatment of Rh immunoglobulin.

Situations in which this may occur include:

This page was last updated on 06/2017

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