Rh factor testing likely wasn’t on your radar before you had a baby on the way. But part of having a healthy pregnancy means getting screened.
The Rh factor is a protein carried by red blood cells in some people and not in others. If you don’t have it, you may need a simple but effective treatment during pregnancy to protect you and your baby.[1]
Here’s what you need to know about Rh factor testing, how the test works and what it means for your pregnancy if you’re Rh negative.
What does Rh factor mean?
Rh factor is a type of protein often found on red blood cells, which carry oxygen throughout the body. But not everyone has it. If you carry the protein, you’re Rh positive. If you don’t, you’re Rh negative.[2]
Not many people are Rh negative. Negative Rh factor occurs in around 15 percent of white people and between 4 and 8 percent of Black people. In Asian people, the chances of being Rh negative are only around 0.3 percent.[3]
Whether you’re Rh positive or negative doesn’t matter much, except when it comes to pregnancy.
What is Rh incompatibility?
Rh incompatibility is when a mom is Rh negative and her baby is Rh positive, which can cause problems during pregnancy.
If the baby’s Rh positive blood cells enter the mother's circulation, her immune system may view them as "foreign" — and, in a normal immune response, mobilize armies of antibodies to attack these unfamiliar cells.
Since red blood cells carry oxygen, this type of attack can make it hard for a baby to get the amount of oxygen she needs. That can potentially cause a serious type of anemia and other health problems.
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It may sound scary, but Rh incompatibility only becomes a problem if you and your baby’s blood mix, most likely during birth. At that point, the antibodies your body makes cannot harm your baby. But they could be dangerous later on if you become pregnant with another Rh positive baby.[4]
Why is Rh factor testing important during pregnancy?
Having Rh incompatibility can set the stage for problems, particularly if you’re planning on having more children later on. That’s why it’s so important to get tested and, if needed, treat the problem.
Fortunately, finding out your Rh status is as easy as taking a blood test. Rh factor screening is typically part of the initial prenatal blood workup. And if you turn out to be Rh negative, the problem is easy to treat.[5]
What happens if you test Rh positive?
If you test Rh positive, as most of the population is, the issue of compatibility is moot. Whether the fetus is Rh positive or Rh negative, there are no foreign antigens on fetal red blood cells to cause your immune system to mobilize against them.
What happens if you test Rh negative?
If you test Rh negative, your baby's father will be tested to determine whether he's Rh positive or negative. If he turns out to be Rh negative, your fetus will be Rh negative too. That’s because two Rh negative parents can't have an Rh positive baby.
If your baby’s father is Rh positive, however, there's a significant possibility that your fetus will inherit the Rh factor from him, creating an incompatibility between you and your baby.
This incompatibility is usually not a problem in a first pregnancy because you don’t have antibodies to the baby's Rh factor yet. However, once a mom's natural protective immune response kicks in and produces antibodies during her first pregnancy or delivery (or abortion or miscarriage), they stay in her system.
That becomes a concern if she becomes pregnant again with another Rh positive baby. During the subsequent pregnancy, these antibodies could potentially cross the placenta into the baby's circulation and attack the fetal red blood cells, causing very mild (if maternal antibody levels are low) to very serious (if they are high) anemia in the fetus.[6]
Rh incompatibility treatment (RhoGAM)
Preventing this antibody response in Mom is the key to protecting the fetus when there is Rh incompatibility. Most practitioners use a two-pronged strategy.
If you're Rh negative at 28 weeks, you'll get an injection of Rho(D) immune globulin (RhoGAM) to prevent antibodies from forming. Another dose is administered within 72 hours after delivery if blood tests show your baby is Rh positive. (If your baby is Rh negative, no treatment is required.)[7]
The RhoGAM injection should also be administered after any genetic testing that could result in mixing of maternal and fetal blood, such as chorionic villus sampling (CVS) or amniocentesis.
Vaginal bleeding or trauma during pregnancy, miscarriage, ectopic pregnancy and abortion are the other situations where fetal blood can get into a pregnant woman's bloodstream, so a RhoGAM injection is given then as well. Getting a RhoGAM shot at these times can head off problems in future pregnancies.
If you're Rh negative, did not get a RhoGAM injection during your previous pregnancy, and you've developed antibodies capable of attacking an Rh positive fetus, amniocentesis or a noninvasive blood test can be used to check the blood type of the fetus (though not all insurance will cover the blood test, since it's pricey).
If the fetus is Rh positive, and thus incompatible with your blood type, ultrasound tests will be done every week or two to assess the baby's condition.
If at any point anemia has developed, a transfusion of Rh negative blood to the fetus may be necessary. This is done through a small needle placed in the umbilical cord under ultrasound guidance. Such transfusions are very effective and associated with excellent outcomes.
Thinking about all this can make you anxious if you and your baby are Rh incompatible. The good news is that once the incompatibility is treated, the chances of having a healthy pregnancy and baby are very, very high. In fact, RhoGAM has reduced the need for transfusions in Rh incompatible pregnancies to less than 1 percent.
Other blood type incompatibilities
A similar incompatibility can arise with other factors in the blood, such as the Kell antigen, though these are less common than Rh incompatibility. If the father has the antigen and the mother does not, there's again the potential for problems.
Part of the first routine blood test looks for the presence of circulating antibodies in the mom's blood. If these antibodies are found, the father of the baby is tested to see if he's positive, in which case the management is the same as with Rh incompatibility.
Rh factor incompatibilities during pregnancy aren’t all that common. But when they do occur, they’re easy to find and treat — so you, your baby and any future children you might have can be protected.
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