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What Blood Type Is Dangerous For Pregnancy? It Actually Depends On Your Baby

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Your first prenatal appointment can be a whirlwind of excitement, nerves — and a lot of tests, too. One of those tests that you’ll be taking is to determine what blood type you are, which might not seem like a big deal, especially if you already know that you’re O-positive, for example. But it might surprise you that there are blood types that can be dangerous for pregnancy, particularly if you and your baby are opposites.

During your first prenatal visit, your blood will be tested for a variety of things, like your blood type, anemia, syphilis, rubella, and Hepatitis B, reported Parents. You’ll also be tested for your Rh factor, which is a protein that’s found on the surface of red blood cells. “If your blood has the RH antigen, then you are Rh positive,” explains Dr. Armando Hernandez-Rey, a reproductive endocrinologist. “Without the antigen, you are Rh negative.”

Being Rh positive (which is the most common blood type, as per Carter BloodCare) or Rh negative typically isn’t a big deal — that is, until you become pregnant. If you’re Rh negative and you’re carrying an Rh positive baby, there can be problems, the Mayo Clinic reported. “During the pregnancy, things can happen and the baby's blood might mix with the mother's blood during that first pregnancy,” Dr. Geoffrey Cly, M.D., FACOG, tells Romper. “The mother's blood develops anti-Rh antibodies, and the next time that woman gets pregnant, her bloodstream will attack the second baby during the pregnancy if that baby also has Rh positive blood type.”

If it sounds kind of scary, it is. “It can be very dangerous for your baby, and may cause problems such as prematurity, growth restrictions, and in severe cases, stillbirth,” says Dr. Cly. (In addition to Rh, another dangerous blood type is Kell, which can occur when a mother has had a blood transfusion in the past.) “The blood that was transfused would have the Kell type and then the woman's bloodstream will develop antibodies Kell antibodies,” says Dr. Cly. In turn, the Kell antibodies could attack a baby during the pregnancy since they don’t recognize the baby’s normal blood type.

It’s important to note that the blood types themselves are not dangerous for pregnancy. “However, the antibodies that can be created by the immune system because of a person’s blood type is what ultimately becomes dangerous for the pregnancy,” reports Dr. Cly. Adds Dr. Hernandez-Rey: “The only danger is not knowing the mother’s blood type during pregnancy.”

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There are things that can be done to help keep your baby safe during your pregnancy if you are Rh negative and your baby is Rh positive. “A maternal fetal medicine specialist can follow your pregnancy closely with ultrasounds to look for growth restriction as well as an amniocentesis sampling of the amniotic fluid which is a way to tell if the Rh antibodies are attacking the baby,” says Dr. Cly. “If the baby is in jeopardy from the Rh antibodies, then the specialist can actually give the baby a blood transfusion during the pregnancy which they might have to do until the baby is far enough along to be delivered, around eight months or 32 weeks or beyond.”

You might also receive an Rh immune globulin injection (RhoGAM) at any point in your pregnancy in which your blood comes into contact with your baby’s blood, including your baby’s birth, according to the Mayo Clinic. “It’s a medication that blocks the formation of antibodies,” says Dr. Hernandez-Rey. “At the time of delivery, within 72 hours of the baby's birth, the mother will be injected with RhoGAM, which eliminates any danger to the baby.”

If you’re pregnant and find that you’re Rh negative and carrying an Rh positive baby, don’t worry too much. With careful monitoring and supervision from your OB, you should have a healthy and event-free pregnancy and delivery. Now that’s definitely a positive.