Rh Incompatibility

Last Updated September 2022 | This article was created by editorial staff and reviewed by Beth Oller, MD

Rh incompatibility is a mismatched blood type between a pregnant mother and the baby she is carrying. It is rarely serious or life threatening, thanks to early diagnosis and treatment during pregnancy.

Rh factor is a protein located in red blood cells. People who have that protein are Rh-positive. Most people are Rh-positive. People without the protein are Rh-negative. You inherit your blood type from your parents.

If an Rh-positive baby’s blood passes to its Rh-negative mother during pregnancy (or delivery), the mother’s body will attack the baby’s red blood cells. Typically, this is not a concern for a live birth with a first pregnancy. It poses a greater risk in later pregnancies. This is because the mother develops antibodies to attack Rh-positive blood types in future children.

Rh incompatibility isn’t harmful to the pregnant mother. However, it can cause mild to serious medical problems for the baby. Doctors treat the condition by injecting the mother with a Rh incompatibility medicine that protects the baby’s red blood cells.

Path to improved health

In most cases, Rh incompatibility is avoidable with preventive care. Your doctor will check your blood type during your first pregnancy visit. If you have Rh-negative blood, you will be given an injection of Rh immunoglobulin. This happens at around week 28 of your pregnancy. It will be done again within 72 hours of your baby’s birth. It may also be done after a miscarriage, an abortion, or an amniocentesis (a gene screening test done during pregnancy). These are all cases in which the mother and baby’s blood could mix.

According to the American Academy of Family Physicians (AAFP), all pregnant women should have blood typing and Rh testing on their first visit to their doctor for pregnancy care. AAFP recommends retesting between the 24th and 28th weeks of pregnancy.

Rh immunoglobulin will not harm your baby. The injection may cause you to have mild soreness around the injection site. For some pregnant women, common side effects of the medicine include

  • Headache
  • Mild fever
  • Mild pain
  • Swelling or redness at the site of the injection

More serious side effects include:

  • Severe allergic reaction
  • Back pain
  • Problems with your urine
  • Rapid heartbeat
  • Nausea
  • Fever
  • Trouble breathing
  • Unexplained weight gain
  • Swelling
  • Fatigue
  • Yellowing of the eyes or skin

Things to consider

Most Rh-positive babies born from a first-time pregnancy to an Rh-negative mother are not affected by Rh incompatibility. This is because the baby’s blood doesn’t usually pass to the mother’s bloodstream until the time of the birth (vaginal or cesarean section). Exceptions may occur if the mother:

  • Had a previous pregnancy that ended in miscarriage or had an abortion.
  • Had pregnancy screening tests, such as amniocentesis or chorionic villus sampling (genetic tests that require inserting a needle into the mother’s womb to sample the baby’s cells).
  • Had bleeding during her pregnancy.
  • Had to have the baby manually rotated from a breech position before her labor started.
  • Experienced a blunt trauma injury to her abdomen during her pregnancy.

Once an Rh-positive baby’s blood enters an Rh-negative mother’s bloodstream, a mother’s future Rh-positive babies are at risk for certain medical problems (unless the mother received an Rh immunoglobulin injection). Without that preventive treatment, Rh incompatibility destroys your baby’s red blood cells (hemolytic anemia) during pregnancy. Red blood cells are filled with iron-rich protein (hemoglobin) that supplies oxygen to your baby. Your baby’s red blood cells die faster than his or her body can make new ones.

Without enough red blood cells, your newborn baby won’t get enough oxygen. The baby could suffer from mild conditions, such as anemia (low blood count) and jaundice (yellowing of the eyes and skin). It could also lead to more serious conditions, such as brain damage and heart failure. It’s possible for a baby to die during the pregnancy if too many of their red blood cells have been destroyed.

Questions to ask your doctor

  • Does my unborn baby’s blood have to be tested during pregnancy or just mine?
  • Does the father’s blood type matter?
  • Is blood typing done on pregnant women of all ages?
  • What happens if I do not receive the final Rh immunoglobulin injection before my baby is born?


National Institutes of Health, MedlinePlus: Rh incompatibility

National Institutes of Health, National Heart, Lung, and Blood Institute: Rh Incompatibility


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