ABO Incompatibility in a Newborn: Why Blood Type Matters in Pregnancy

Finding out that you and your baby have different blood types can sound scary. But a blood type mismatch usually doesn't cause serious health problems. ABO incompatibility happens most often when a mother has Type O blood and her baby has Type A, B, or AB blood. 

Most cases are mild and easy to treat. Here's what ABO incompatibility means, how your healthcare team checks for it, and what to expect if your baby needs treatment. 

What is ABO incompatibility in a newborn?

ABO incompatibility happens when a pregnant mother and baby have different blood types and the mother’s immune system reacts to the baby’s blood. The mother has antibodies that can cross the placenta and break down some of the baby’s red blood cells. 

This can cause hemolytic disease of the newborn (HDN). HDN from ABO incompatibility is usually mild and most often causes jaundice. More serious anemia can happen, but it’s very rare.

What causes ABO incompatibility in a newborn baby?

ABO incompatibility almost always happens when a mother has Type O blood and her baby has Type A, B, or AB blood. People with Type O blood naturally make antibodies against the A and B proteins found on red blood cells. These antibodies can cross the placenta and attack a baby's Type A or Type B red blood cells.

This blood type mismatch is fairly common. It occurs in about 15% to 20% of pregnancies. But only about 1% to 4% of these pregnancies lead to a case of HDN. So having different blood types doesn't mean your baby will develop HDN.

Who is at risk for ABO incompatibility?

ABO incompatibility is mainly a concern if you have Type O blood and your baby has Type A, B, or AB blood. If you and your baby both have Type O blood, there isn't an ABO mismatch.

You can't control your blood type or your baby's blood type. And there isn't anything you can do during pregnancy to cause ABO incompatibility or HDN. 

Keep in mind that most Type O mothers with a Type A, B, or AB baby won’t have a baby who develops HDN.

What are the signs and symptoms of ABO incompatibility?

Most babies with HDN from ABO incompatibility have mild symptoms. These can include:

• Jaundice: Jaundice during the first 24 hours after birth is the most common sign. It may be the only symptom. Jaundice happens when red blood cells break down and release bilirubin into the blood. Babies with high bilirubin may need light therapy (phototherapy).

• Anemia: Some babies develop mild anemia, which means they have fewer red blood cells than normal. It usually doesn't cause health problems or need treatment.

Rarely, ABO incompatibility causes more severe HDN. This can lead to very high bilirubin levels or severe anemia that needs more treatment.

Some biological factors can make severe HDN more likely. These include:

• Higher antibody levels in the mother: More anti-A or anti-B antibodies can cause more of the baby's red blood cells to break down.

• The baby's blood type: HDN may be more severe if the baby has Type B rather than Type A blood — when the mother has Type O blood.

• Proteins on the baby's blood cells: Some babies naturally have more A or B proteins for the mother's antibodies to attack.

• Other conditions that increase jaundice: Conditions such as G6PD deficiency and certain genetic differences can make high bilirubin levels more likely.

You can't control or change these factors. But knowing about them can help your healthcare team decide if your baby needs closer monitoring. This can help them spot more serious HDN early and start treatment quickly if needed.

How is ABO incompatibility diagnosed?

Blood typing during pregnancy can help identify who may be at risk for ABO incompatibility. For example, if you have Type O blood, your healthcare team will know that your baby could be at risk if they have Type A, B, or AB blood.

After birth, your baby’s healthcare team will watch for signs of HDN. This may include:

• Checking for jaundice: The healthcare team will look for yellowing of your baby’s skin or eyes.

• Measuring bilirubin through the skin: A transcutaneous bilirubin monitor can quickly estimate your baby’s bilirubin level without drawing blood.

• Doing blood tests: If needed, a blood test can measure bilirubin more accurately. A complete blood count (CBC) can also check for anemia and other signs that red blood cells are breaking down. Checking your baby’s blood type will also help identify if they’re at risk for developing HDN. 

Your healthcare team will monitor how bilirubin and red blood cells levels change over time to decide if treatment is needed.

How is ABO incompatibility in a newborn treated?

Most cases of HDN from ABO incompatibility are mild. Some babies don't need treatment beyond regular newborn care and monitoring.

If bilirubin levels get too high, your baby may need phototherapy. This treatment uses special light to help the body get rid of bilirubin. About 1 in 3 babies with HDN from ABO incompatibility need phototherapy. And about half of those babies start it within the first 24 hours after birth. Most babies who need phototherapy receive it for about 1 to 2 days.

Bilirubin can rise again after phototherapy stops because red blood cells may continue to break down. Your baby may need repeat blood tests to make sure their bilirubin stays at a safe level. This may mean staying in the hospital longer or seeing your pediatrician sooner and more often after you go home.

Very rarely, ABO incompatibility causes severe HDN that needs more treatment. These babies may need care in the neonatal intensive care unit (NICU). Treatments can include IV immune globulin (IVIG), a red blood cell transfusion, or an exchange transfusion. These treatments are rarely needed for HDN caused by ABO incompatibility.

How can you reduce the risk of ABO incompatibility in newborns?

There’s nothing you can do to prevent ABO incompatibility because you can’t change your blood type or your baby’s blood type. But keeping up with prenatal care can help your healthcare team know if your baby may be at risk for HDN. After birth, your baby may need closer follow-up and repeat bilirubin checks. This can help catch jaundice early so your baby can get treatment before bilirubin levels get too high. 

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Written by:

Alicia Stevens, MD

Alicia Stevens, MD, is a board-certified physician in the fields of internal medicine and pediatrics with over 15 years of experience. She graduated from the University of Cincinnati College of Medicine and completed her training at Tulane University.

Edited by:

Patricia Pinto-Garcia, MD, MPH

Patricia Pinto-Garcia, MD, MPH, is a medical editor at GoodRx. She is a licensed, board-certified pediatrician with more than a decade of experience in academic medicine.

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