ABO Incompatibility

ABO incompatibility is when you’re pregnant and the fetus has an ABO blood type that doesn’t match yours. The mismatch can cause your body to have a mild reaction to the fetus’s blood. It’s most common if you have type O blood and the fetus has type A, B or AB blood. It happens in about 15 out of 100 pregnancies.

Rarely, blood group mismatches can cause the mother’s immune system to attack the fetus’s red blood cells. This is called hemolytic disease of the newborn (HDN). Affected babies have anemia and jaundice.

The good news is that ABO incompatibility issues are extremely rare, happening in under 1% of pregnancies. And if there are problems, there are treatments that can keep you and your baby safe.

Symptoms of ABO incompatibility

Babies born with ABO hemolytic disease may not have symptoms. Sometimes, they have jaundice that starts within the first 24 hours of birth. This is a sign that some of their red blood cells are breaking down too soon. When they break down, red blood cells release a substance called bilirubin that makes skin look yellow.

But many infants are born with jaundice. Whether your baby needs treatment depends on how severe it is.

ABO incompatibility causes

ABO incompatibility happens when the mom has one blood type within the ABO blood group (A, B, AB or O) and the fetus has a different type. Each blood type has unique antigens on red blood cells. Antigens are like name tags that let your immune system know that a red blood cell belongs. A has “A” antigens on red blood cells. B has “B” antigens on red blood cells, and so forth. Your immune system won’t attack if it recognizes an antigen.

If the fetus’s blood cells have an antigen your immune system doesn’t recognize, it may flag them as a threat. It may make antibodies that attack the fetus’s red blood cells.

But ABO incompatibility doesn’t usually cause your immune system to launch harmful attacks. This happens more with “Rh incompatibility.” Rh factors determine whether you have a positive or negative blood type. The harmful attacks with Rh incompatibility can happen when the mother has blood type with Rh- and the fetus has Rh+ blood type.

If there’s a chance that you and the fetus have Rh incompatibility, you can take medicines to keep the fetus safe.

How doctors diagnose this condition

In the U.S., screening for blood type incompatibility is part of routine prenatal care. This lets your healthcare provider know if there’s a chance of blood type incompatibility that may pose risks to your pregnancy.

If your baby is born with signs of disease, your healthcare provider may do tests to check their:

• Blood type, to see if blood type incompatibility may be what’s causing issues
• Bilirubin levels, to see if their levels are so high that they need treatment
• Levels of antibodies, to see if your immune system is launching attacks
• Levels of red blood cells, to see if they have anemia

If your baby is born with signs of severe disease, your provider will look for causes other than ABO incompatibility.

How is it treated?

Neither you nor the fetus will likely need treatment. If your baby develops jaundice, their healthcare provider may suggest phototherapy. This treatment uses safe UV (ultraviolet) lights to help your baby’s body get rid of excess bilirubin.

What can I expect if I have this condition?

ABO incompatibility rarely causes problems during pregnancy. With other blood type incompatibilities — like those related to Rh factor — your provider may need to closely monitor your pregnancy. You may need to take medicines to keep the fetus safe.

The most important thing to know is that providers go to great lengths to keep pregnancy safe for you and your baby. A blood type incompatibility doesn’t prevent most women from delivering healthy newborns.