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What is Rhogam and why do Pregnant Women Need It?

Rhogam, or anti-D immune globulin, is an injection given to certain pregnant women. It\’s designed to help prevent immune-system problems related to your blood type and whether it\’s \”negative\” or \”positive.\” For example, you might have \”A positive\” or \”A negative\” blood type. The negative/positive refers to the absence or presence of Rh antigen on the surface of your red blood cells. Women who are Rh positive have the antigen. Women who are Rh-negative do not have the antigen. Problems arise when an Rh-negative woman is carrying an Rh-positive baby. This condition is called Rh incompatibility.

If a mother does not receive Rhogam and goes on to deliver an Rh-positive baby, at the time of delivery some of the baby’s blood will enter the mom’s bloodstream. Her body has now been exposed to red blood cells that have the Rh antigen. Since her own body does not carry that antigen, her body mounts an immune attack and makes antibodies, much like how your body would fight off an invading virus. While her baby has safely delivered, it is her next pregnancy that is now in danger and will suffer from what is called Rh sensitization.

Fast forward to her next pregnancy (assuming she’s still with her Rh-positive male partner). Those antibodies from her first delivery are still present, and they can cross the placenta and start to attack the Rh antigens on the developing fetus. This destroys the fetus’ red blood cells and leads to a condition called hemolytic anemia, which gone untreated can be fatal. Before the invention of Rhogam, this condition complicated 10 percent of all pregnancies and led to the deaths of thousands of babies.

So how does Rhogam prevent this? When Rhogam is injected into a pregnant woman, it will seek out any red blood cells that express the Rh antigen (such as from her unborn or just-born child) and bind to them, essentially “hiding” them from her immune system. In this way, her body doesn’t detect they are present and won’t make antibodies. This means her next pregnancy won’t suffer from this potentially fatal complication.

All Rh-negative women receive Rhogam under circumstances where her blood might mix with fetal blood, including:

  • At any time in pregnancy if she has vaginal bleeding
  • If she miscarries or has an ectopic pregnancy
  • If she undergoes a procedure such as an amniocentesis

Rh-negative moms also routinely receive it at 28 weeks of pregnancy (to prevent any sensitization for the rest of the pregnancy). Once delivered, the baby will also be tested and if that baby is Rh-positive, one more dose of Rhogam is given.

If a woman chooses to forego Rhogam, she has an approximate 15 percent chance she will become sensitized and thus have issues in her next pregnancy. It only takes 0.1 milliliters of fetal blood to get into a mother’s bloodstream to cause this antibody reaction.


  • The American Congress of Obstetricians/Gynecologists
  • Practice Bulletin # 4: Prevention of Rh alloimmunization.
    The American Congress of Obstetricians/Gynecologists
  • FAQ #27: The Rh factor: How it can affect your pregnancy.

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