Perinatology.com
Patient handout

Red Blood Cell Antibodies in Pregnancy

Also called red cell alloimmunization or Rh disease

A blood test found an antibody in your blood. An antibody is made by your body's defense system. It helps your body fight things that do not belong. Some antibodies can stick to a baby's red blood cells and cause anemia. Anemia means the baby does not have enough red blood cells. Many antibodies do not harm the baby. Your care team will find out which antibody you have and whether your baby needs more tests.

What is alloimmunization?

Red blood cells have tiny markers on their surface. These markers are called antigens. You may not have a certain antigen, but your baby may have it. During pregnancy or birth, a small amount of the baby's blood may mix with your blood, or a blood transfusion can also expose you to an antigen you do not have. Your body may then make antibodies against that antigen.

Antibody Matching marker Red blood cell
An antibody may attach to a matching marker on a red blood cell.
An antibody is in the mother's blood
It may cross the placenta
It may break down the baby's red blood cells

If your baby has the matching marker, the antibody may break down the baby's red blood cells. This can cause anemia. Anemia means the baby does not have enough red blood cells to carry oxygen.

What happens next?

  • If it has not done so already, the lab will find the name of the antibody.
  • A blood test may check how much antibody is present. This is called a titer.
  • Tell your care team about any past baby with anemia, severe jaundice, or a blood transfusion. Also share any pregnancy loss or past blood transfusion.
  • A test may find out if the baby has the matching marker.

A blood test from you may show the baby's marker. A blood test from the baby's father may also help. At times, your doctor may offer amniocentesis.

What if the baby is at risk?

You may need more blood tests and ultrasounds.

A middle cerebral artery (MCA) Doppler ultrasound may be needed. It checks blood flow in a small artery in the baby's brain. Blood that moves too fast may be a sign of anemia. This scan does not hurt you or the baby.

If the baby does not have the matching marker, that antibody cannot harm the baby.

Can anemia be treated?

Yes. If the baby may have severe anemia, you will be sent to a special baby care center.

The team may test a small sample of the baby's blood. A baby with severe anemia may need a blood transfusion before birth. In some cases, delivery may be safer.

What may happen after birth?

Your baby may need blood tests. Some babies need light treatment for yellow skin, called jaundice.

A few babies need new blood by transfusion. Your baby's doctor will make a care plan before birth.

Medical references
  1. Society for Maternal-Fetal Medicine. Clinical Guideline #8: The fetus at risk for anemia”diagnosis and management. Am J Obstet Gynecol. 2015;212:697-710. Reaffirmed 2025.
  2. American College of Obstetricians and Gynecologists. Practice Bulletin No. 192: Management of Alloimmunization During Pregnancy. Obstet Gynecol. 2018;131:e82-e90.
  3. Moise KJ Jr, Markham KB, Spinella PC, et al. A Clinical Practice Guideline for the Management of Pregnancy Alloimmunized to Red Blood Cell Antigens. JAMA Netw Open. 2025;8:e2544649.

Updated August 4, 2026. This handout does not replace advice from your own clinician.