If you’re pregnant or planning to become pregnant, you may have heard about blood type testing. But there’s one part of that test that often raises questions: The Rh factor. 

Here’s what it means, why it matters during pregnancy and how a simple shot can protect your future pregnancies.

What is Rh factor?

Your blood type has two parts: A letter (A, B, AB, or O) and a positive or negative sign. That positive or negative refers to your Rh factor – whether or not your blood contains a specific protein.

“If you have the Rh protein in your body, then you are Rh positive. If you do not have the Rh protein, then you are Rh negative,” says certified nurse midwife Diane Johnson, APRN-CNM.

When and why is Rh factor tested?

Rh factor is checked at your very first prenatal appointment, along with your general blood type. 

“We test it right at the beginning of pregnancy,” Johnson says.

If you are Rh positive, Rh factor is not a concern for your pregnancy. But if you are Rh negative and your baby turns out to be Rh positive, your body may respond to your baby’s blood as if it were a foreign invader – especially if any fetal blood mixes with yours, which can happen during delivery.

“The pregnant person’s body recognizes that the blood cells of the baby are different, so they are like a foreign invader, and then they make antibodies against it,” Johnson says.

Those antibodies don’t typically cause problems in a first pregnancy. But in a future pregnancy, if that baby is also Rh positive, the antibodies can cross the placenta and attack the baby’s red blood cells. This is called Rh incompatibility, and it can lead to a condition called hemolytic disease of the newborn. This can cause:

• Anemia.
• Jaundice.
• Heart failure.
• Brain damage.
• Other serious complications or even death.

What is the RhoGAM® shot?

RhoGAM® is an injection given to Rh-negative pregnant patients to prevent their bodies from developing those antibodies. It is typically given at 28 weeks as a precaution, and again after delivery if the baby’s blood type confirms they are Rh positive.

Johnson notes that the 28-week dose is preventive – at that point, your baby’s blood type is still unknown. The post-delivery dose is given once testing confirms the baby has a different blood type.

RhoGAM® may also be recommended at other times during pregnancy when there is a higher risk of fetal and maternal blood mixing – such as after trauma to the abdomen, significant bleeding or certain medical procedures. 

It is generally not given before 12 to 13 weeks, since there is not enough fetal blood at that stage to be a concern.

Are there reasons someone might decline RhoGAM®?

Because RhoGAM® is technically classified as a blood product, some patients prefer not to receive it. Johnson says this is rare, and that the shot itself carries very little risk – its purpose is protective, not reactive.

One situation where RhoGAM® may not be needed: If a patient’s partner is also Rh negative, the baby is very likely to be Rh negative as well, and the shot may be declined with that understanding.

Common questions

Can Rh factor change over time? No. Your Rh factor does not change.

Can Rh factor cause birth defects? No. Rh factor itself does not cause birth defects.

Is Rh factor inherited? Yes. Your blood type – including your Rh factor – comes from both of your biological parents, similar to other traits you inherit.

The bottom line

Rh factor is a routine part of prenatal care, and for most people, it requires nothing more than a simple blood test at your first visit. 

If you are Rh negative, talk with your provider about the RhoGAM® shot and what to expect. With proper monitoring and treatment, Rh incompatibility is highly preventable.