What is Rhesus Factor: How Does it Affect Pregnancy

What is Rhesus Factor?

Updated on July 9th, 2021 // by Katie Griffin

You may have heard that one of the tests done during pregnancy is an Rh factor blood test. What does this mean? Let’s talk about what rhesus factor is and what its place is in pregnancy.

Estimated reading time: 5 minutes

What is Rhesus Factor?

I’m sure you’ve heard people talk about having blood types that are positive or negative. For example, “My blood type is A positive,” or “I’m B negative.” The positive or negative in this case refers to whether or not you have the Rhesus factor, or Rh factor. (Fun fact: It is called the rhesus factor because it was first detected in the blood of rhesus monkeys!) The Rh factor is an antigen, or tiny protein, that is sometimes found on the surface of red blood cells (1). You are Rh-positive if you have the Rh factor — most people fall into this category. You are Rh-negative if you do not have the Rh factor — only about 15% of the white population and 8% of the African American population are Rh-negative (2).

What is the Rhesus factor test? No worries, this is easy! You’ll have blood drawn at your first prenatal visit, and the Rh factor is one of the tests they’ll run on that blood.

How Does Rhesus Factor Affect Pregnancy?

As we said above, most people are Rh-positive. For women who fall into this portion of the population, there are no worries. The possible rhesus factor complications arise when a woman who is Rh-negative is carrying a baby who is Rh-positive. A baby of an Rh-negative mother may be Rh-positive if the father is positive; two Rh-negative parents can only produce a negative baby. A pregnancy is said to have an Rh incompatibility if mom is negative and baby is positive. This isn’t a situation that should cause excessive worry; your provider will just want to know so that you can take proper precautions.

What Is Rh Incompatibility?

Since a woman who is Rh-negative doesn’t have the rhesus factor in her own blood, her body considers it a foreign material and may make antibodies that circulate in her system. This does not harm the mother, but it could attack the blood of a baby who is Rh-positive. Again, incompatibility isn’t always a problem, so there’s no need for worry, even if you’re Rh-negative.

Mom and baby don’t share blood during pregnancy; rather, nutrients, waste products, etc. pass back and forth through the placenta. However, in some situations, blood from baby can pass to the mother, and the mother’s body may react as if she’s allergic to it (2). In this case, the antibodies from the mother would cross the placenta and break down baby’s red blood cells, leading to anemia. These cells may be destroyed faster than baby’s body can replace them, and because red blood cells carry oxygen, baby may not get enough oxygen (3).

The antibodies that mom’s body produces against the rhesus factor may not be produced quickly enough to affect the baby she’s currently carrying, but the risk would be greater in future pregnancies. For this reason, treatment will be given in the first pregnancy to prevent the mother’s body from making the antibodies to begin with.

Mom is Rh Negative: Precautions

If you’re Rh-negative, your provider will use blood tests to determine whether or not you’re already sensitized, meaning that you have the antibodies against the rhesus factor already in your blood. If you’re not already sensitized, there is a treatment to help prevent you from becoming sensitized. In other words, medication can help prevent your body from making the antibodies that can harm your baby. The treatment is called RhoGAM or RhIg and is usually given around week 28 of pregnancy because your chances of exposure to baby’s blood are highest in the last trimester and during delivery (2). You may also receive another RhoGAM injection after delivery if baby is Rh-positive.

If you are Rh-negative and early pregnancy tests reveal that you already have the antibodies that fight the rhesus factor, your provider will monitor your pregnancy closely. Your provider will watch for signs that baby is developing anemia. If this is the case, your doctor or midwife may decide that the safest option is to induce labor and deliver baby. If it’s too early for baby to be born safely, it is possible to give a blood transfusion through the umbilical cord while baby is still in the uterus (3). Baby may also need a blood transfusion after birth.

Should I Be Worried?

Admittedly, this can all sound a bit overwhelming or even scary. But know that Rh incompatibility isn’t very common. Only around 15% of women are Rh negative, and only some of their pregnancies will have an incompatibility. Even when an incompatibility is present, most pregnancies won’t have complications. Yes, your caregiver may classify your pregnancy as, but even most high-risk pregnancies result in healthy babies. As always, educate yourself, and communicate with your provider to help you make the best choices for you and your baby.

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  1. The American College of Obstetricians and Gynecologists.  (2010).  Your Pregnancy and Childbirth Month to Month, 5th edition.
  2. Glade, B.C., Schuler, J.  (2011).  Your Pregnancy Week by Week, 7th edition.  First Da Capo Press.
  3. Women’s Health Care Physicians. (n.d.). Retrieved from https://www.acog.org/Patients/FAQs/The-Rh-Factor-How-It-Can-Affect-Your-Pregnancy?IsMobileSet=false

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Meet Katie Griffin

I’m a registered nurse, Lamaze certified childbirth educator, and the mother of 7. I help women realize their dream of a natural, intimate, and empowering hospital birth.

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A rhesus factor, or Rh factor, blood test is done during pregnancy. What is its affect on pregnancy and what is Rh incompatibility?