People typically have one of four blood types: A, B, AB or O. Health care providers also classify blood types as positive or negative. For example, your blood type could be type A-negative (A-) or type O-positive (O+).

The presence of a protein (Rh factor) on your red blood cells determines whether your blood type is positive or negative. People with the protein are Rh-positive; those without the protein are Rh-negative.

Blood type incompatibility during pregnancy means that you and your unborn baby have different blood types. As a result, your immune system may react to your unborn baby’s blood type. It may attack your unborn baby’s red blood cells, which can lead to serious problems.

ABO incompatibility is a maternal-fetal blood group problem that can develop during pregnancy. Usually, the mother has type O blood, and her unborn baby has type A, B or AB blood.

Rh incompatibility (also called Rhesus disease) is another blood type issue that may impact your pregnancy. Most people (about 85%) are Rh-positive. But if you are Rh-negative and your unborn baby is Rh-positive, problems may develop during that pregnancy or future pregnancies.

People with blood type incompatibility need treatment during pregnancy to prevent complications. 

Rh incompatibility may not cause problems if you receive treatment before antibodies develop. But left untreated, the condition can lead to dangerous pregnancy complications and fetal conditions such as hemolytic disease of the newborn (HDN). HDN is a blood disorder that affects newborns.

You inherit your blood type, meaning it’s passed down to you from your mother or father. Blood type incompatibility occurs when you have a different blood type than the baby developing in your womb.

If incompatible blood types mix during pregnancy, your immune system makes antibodies that attack your baby’s blood cells. This reaction, called Rh sensitization, is similar to how your body protects itself from illnesses or allergens.

A mother and baby’s blood mix during labor and delivery. So these antibodies usually don’t develop until after birth. Incompatibility problems are more likely to occur during a second pregnancy because the antibodies are already present.

Left untreated, Rh or ABO incompatibility during pregnancy can be very dangerous for an unborn baby or newborn. The mother’s antibodies attack the baby’s red blood cells and deprive the baby of oxygen, which can be life-threatening.

There are some risk factors for blood incompatibility, including:

An Rh factor test determines whether you are Rh-positive or negative. It is a simple blood test you receive at the beginning of your pregnancy, usually at your first prenatal care visit.

If you are Rh-negative, your provider will talk to you about when you should begin treatment to prevent antibodies. Your provider may also recommend other antibody screening tests throughout your pregnancy.

If antibodies are present, you might need extra blood tests and ultrasounds during your pregnancy to monitor your baby’s health. At UC Davis Health, we have specialized expertise evaluating and monitoring Rh compatibility and other types of high-risk pregnancies. Our team provides the care needed to keep your unborn baby as healthy as possible.

At UC Davis Health, experts across several disciplines work together to safeguard the health of you and your baby. You receive seamlessly coordinated care from top obstetricians, maternal-fetal medicine specialists and neonatologists (newborn specialists).

You can’t control your blood type or the blood type of your baby. But you can take measures to prevent Rh disease if you and your baby have incompatible blood types. If you are Rh-negative, seek treatment during your pregnancy to prevent antibodies from developing.