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What Is ABO Incompatibility

What happens when babies and parents have different blood type?

What Is ABO Incompatibility?

ABO incompatibility is a condition in which a newborn's blood type is different from the birthing parent's blood type. It is also called ABO blood type incompatibility, and is a type of illness known as a hemolytic disease of the newborn (HDN).1 If the baby's blood mixes with the parent's, the baby's body may attack the red blood cells and break them down.

In the past, an HDN (clinically known as erythroblastosis fetalis) could put a baby's health at serious risk. In fact, at one time HDNs were a major cause of death for new babies. Now that medical researchers better understand the mechanisms that cause HDNs, they aren't nearly as threatening.

Another example of an HDN occurs when a gestational parent's blood is Rh Negative and their baby is Rh-positive. Rh factor incompatibility is screened for during pregnancy; an expectant parent can be given a special shot to prevent problems.2 This HDN is more serious than ABO incompatibility.

Symptoms of ABO Incompatibility in Newborns

There are two main signs that a newborn has an ABO incompatibility: jaundice and anemia.


Jaundice is the most common problem caused by ABO incompatibility. Jaundice occurs when an orangish-red substance called bilirubin builds up in the blood. Bilirubin is produced when red blood cells break down.

If more red blood cells are broken down at once than is normal, the bilirubin that results will deposit fatty tissue under the skin, causing it to take on a yellowish hue. Other signs and symptoms of jaundice include yellowing of the whites of the eyes, sleepiness, and poor feeding.


Anemia may not always occur with ABO incompatibility, and it is usually mild.4 Sometimes it has no symptoms. Or, a newborn may have symptoms such as low energy, difficulty feeding, pale skin, and a fast heart rate and rapid breathing while resting.5


To diagnose ABO incompatibility in a newborn, doctors can use a blood test called the Coombs test it can show whether the baby's blood has antibodies from the parent in it. Or, sometimes a parent's prenatal blood screening tests can reveal ABO incompatibility.4

If the baby is showing signs of jaundice, a blood test can check the levels of bilirubin in the baby.3

Causes of ABO Incompatibility

Blood type is based on genes from each parent. So if one parent is type O and one is type A, the baby will likely be type A.6 The reason the baby wouldn't have type O blood is because the gene for O is recessive (meaning it's only expressed if the baby gets it from both parents).

The four blood types are A, B, AB, and O. Blood type is determined based on proteins on the surface of red blood cells. These proteins are potential antigens—substances the immune system doesn't recognize. The immune system creates antibodies to fight off the unfamiliar protein. These antibodies can cross the placenta, where they break down the baby's red blood cells after birth.1

Not all non-matching combinations of blood types are problematic. ABO incompatibility in newborns usually only happens if a gestational parent with type O blood has a baby whose blood is type A, type B, or type AB. Type O blood has anti-A and anti-B antibodies, which cause the incompatibility.4

Adults can also experience ABO incompatibility if they receive a blood transfusion or transplant of the wrong type of blood.7


Not every baby with ABO incompatibility will require treatment. Often, anemia resolves on its own without treatment, or with changes in diet.5 And not all babies develop jaundice, or they have jaundice that is mild and doesn't require extensive treatment. It will depend on how much bilirubin collects in the baby's blood.

Extra Feedings

Some infants with mild jaundice will get better on their own simply by being fed more often.3 A temporary increase in feedings will lead to an increase in bowel movements, which is how excess bilirubin exits the body. Parents who breastfeed may need to supplement their baby's diet with a formula for a few days8 if nursing alone doesn't do the trick.


For infants whose jaundice is more severe, phototherapy, or light therapy, is effective. The baby's skin is exposed to light waves that transform the bilirubin into a substance that can pass through the baby's system. The baby rests under special lights wearing just a diaper and soft eye patches.9

Instead of, or in addition to, phototherapy, a baby with jaundice may be treated with a bili blanket which uses fiber optics to break down bilirubin.

Blood Transfusion

In rare cases, a baby with an HDN will need to be treated with a type of blood transfusion called an exchange transfusion.1 This is when a portion of a baby's blood is replaced with type O blood.

A child who becomes severely anemic as a result of an HDN may need a more traditional Blood transfusion in which they receive extra blood to replace blood that's lost.

A Word From Verywell

ABO incompatibility in newborns is usually not a serious condition. If your baby develops jaundice as a result of ABO incompatibility, extra feedings and phototherapy are simple and effective treatments.


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