What is Rh factor?

Rh factor, also called Rhesus factor, is a type of protein found on the outside of red blood cells. The protein is genetically inherited (passed down from your parents). If you have the protein, you are Rh-positive. If you did not inherit the protein, you are Rh-negative. The majority of people, about 85%, are Rh-positive.

Why is Rh factor important?

This protein does not affect your overall health, but it is important to know your Rh status if you are pregnant. Rh factor can cause complications during pregnancy if you are Rh-negative and your child is Rh-positive.

What is Rh incompatibility?

Rh incompatibility occurs when a woman who is Rh-negative becomes pregnant with a baby with Rh-positive blood. With Rh incompatibility, the woman’s immune system reacts and creates Rh antibodies. These antibodies help drive an immune system attack against the baby, which the mother’s body views as a foreign object.

Antibody formation can happen after blood transfusions or when fetal blood enters the mother’s circulation:

Who is at risk for Rh incompatibility?

A woman who is Rh-negative is at risk for Rh incompatibility when she becomes pregnant. Rh incompatibility happens only when the father of the baby is Rh-positive. Doctors do not routinely test men’s Rh status. Instead, expectant parents discuss their individual status with their doctor.

What causes Rh incompatibility?

A difference in blood type between a pregnant woman and her child causes Rh incompatibility. Children may be Rh-positive if they inherit the protein from their father, even if their mother is Rh-negative.

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