Why is Rh Factor Important in Pregnancy?

During your pregnancy, early on in you will have a test to see if you are Rh negative. But what is the Rh factor and why does it play such a big role in pregnancy? 

The rhesus (Rh) factor is a protein antigen found on some surfaces of red blood cells. Not everyone carries this protein, but if you do, then you are Rh-positive. Those who don’t carry the protein are Rh-negative. Only about 15 percent of the population is Rh-negative, with its occurrence in other ethnic groups significantly less. 

If you are Rh-positive, your blood has the necessary components to match the Rh-positive blood of your baby. However, if you are Rh-negative and your fetus is Rh-positive, your immune system will work to develop antibodies against your baby. Often the first baby can survive even with this incompatibility, but subsequent pregnancies often end in miscarriage for women who are Rh-negative and do not get Rhogam.  

Treatment for Rh-Negative Blood Type 

In the U.S., when you are pregnant you are routinely checked for Rh negativity at about 20 weeks. If you test Rh-negative, you will be given an injection of Rhogam at about 28 weeks. If both you and your partner are Rh-negative, your baby will also be Rh-negative. However, if you are Rh-negative, often you will be given a Rhogam shot without further testing of your partner.  

The danger of the blood incompatibility occurs whenever you undergo a procedure which provides the opportunity for your blood to become mixed with your baby’s, such as amniocentesis, including the birthing process itself.  For this reason, if you are Rh-negative, another shot of Rhogam is given within three days after delivery of your baby.  

What Does Rhogam Do? 

Your immune system produces antibodies to what it considers invading substances. If you are Rh-negative and your body produces antibodies to your baby, they can sometimes reach your baby. During a first pregnancy, your baby is likely to survive. When your first baby is born, there is ample opportunity for the mixing of small amounts of your blood with your baby’s. Your natural antibodies will have what is called memory, and the next time you are pregnant, your antibodies will be ready to attack your baby if there is any opportunity for your and your baby’s blood to mix.  

All that is needed to cross into the mother’s blood is 1 to 2 milliliters of Rh-positive blood for the mother to make memory cells. This can happen, for example, if you have an amniocentesis. Your first baby may escape the damage done by the antibodies if there is no mixing of your blood with your baby’s, but your second baby will be exposed to your already existing antibodies.  

Rhogam provides what are called “passive” antibodies. They cause your body to think it already has the antibodies needed. Rhogam antibodies leave no “memory” of your being exposed in your immune system. This prevents your body from developing the natural antibodies which would later cause difficulty in subsequent pregnancies. 

The last 50 years has seen the development of many treatments for pregnant women to make their deliveries successful. The introduction of Rhogam in the 1960s is no doubt one which has been a tremendous benefit to pregnant women who are Rh-negative and their babies.

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Dr. Alan Lindemann

Obstetrician-Gynecologist (OB/GYN)​

He is an obstetrician and maternal mortality expert with 4 decades of medical practice beginning in Minnesota and presently in North Dakota. He has delivered around 6,000 babies with zero maternal deaths.