How is maternal alloimmunization treated during pregnancy?

On rare occasions, a pregnant person’s blood may be incompatible with the blood of their growing baby.

That can put the baby at risk of anemia and even death.

What is maternal alloimmunization?

Alloimmunization is a condition that occurs in 1% to 2% of pregnancies. It happens when the immune system of a pregnant individual reacts to proteins on the red blood cells of their growing baby as if they were foreign.

As a result, the pregnant parent’s immune system develops antibodies − proteins that can cross the placenta and attack the baby’s red blood cells.

A blood test early in pregnancy can detect these antibodies.

What causes maternal alloimmunization?

Most often, the incompatibility in the blood of parent and fetus happens because of a protein found on the outside of red blood cells called the Rhesus factor, or Rh factor.

People who have the Rh factor on their red blood cells are Rh-positive. People who don’t have it are Rh-negative.

If a pregnant person is Rh-negative and their growing fetus is Rh-positive, alloimmunization may be prevented by giving the pregnant person a shot of a prescription medicine called RhoGAM before they develop antibodies harmful to their growing fetus.

How is maternal alloimmunization treated during pregnancy?

Once a pregnant person has already begun making antibodies, if those antibodies cross the placenta in high enough quantities and begin to destroy the red blood cells of the fetus, there’s no treatment that can reverse that process.

But a lot can be done to keep the baby safe even as the pregnant parent’s immune system continues to attack the baby’s red blood cells.

Here's what we can do to protect the baby:

Before birth

• Fetal ultrasounds can monitor the baby and check for signs of anemia.
• Blood transfusions can be given to the baby through the umbilical cord, to treat anemia.
• Early delivery may be needed, ideally after 24 weeks, when the baby’s odds of surviving are better than earlier in the pregnancy.

After birth

• Monitoring in the neonatal intensive care unit (NICU)
• Blood transfusions may be needed.

How are blood transfusions given to a baby in the womb?

A fetal ultrasound guides a maternal-fetal medicine physician while they insert the needle through the parent's abdomen and into the umbilical cord. Sometimes the baby will move around during the procedure, which can be challenging.

Once the needle is placed in the umbilical cord, donor blood is given to the baby.

Healthcare providers calculate how much blood to give the baby based on the baby’s size and how anemic the baby is. A baby who receives too much blood at once is at risk of heart failure.

After birth, how healthy are babies whose mothers had maternal alloimmunization?

Babies properly cared for during pregnancy and immediately after birth can go on and be healthy. They may need one or more additional blood transfusions after they’re born.

Within three months after birth, typically the pregnant parent’s antibodies are no longer present, and the baby’s red blood cells are no longer being attacked.

Can maternal alloimmunization be prevented?

Alloimmunization can sometimes be prevented. The incompatibility between proteins on a pregnant person’s red blood cells and those of the baby’s red blood cells can happen for several reasons.

Most cases of alloimmunization involve an incompatibility of the Rh-factor. For those, a RhoGAM shot may prevent the harmful immune response.

But there are some other incompatibilities that involve other proteins. For those, there are no shots that can stop a pregnant person’s immune response.

While alloimmunization can be life-threatening to the fetus, proper screening in pregnancy and appropriate surveillance can significantly lower the risk of pregnancy loss.