A woman’s blood type and Rh factor can impact her pregnancy. That’s why every pregnant woman will have an Rh factor test. In some cases, the father of the baby will be tested too. Here’s what you need to know about blood types and Rh factor.

Blood Types

The four major blood types are:

  • A
  • B
  • AB
  • O

Which classification you are is determined by antigens (proteins) on the surface of blood cells that trigger an immune response.

According to the American Pregnancy Association, the Rh factor is a specific type of protein on the surface of red blood cells. If your blood has these proteins, it’s considered Rh positive. This is the most common blood type.

If you don’t have these proteins, you are Rh negative. Being Rh negative is not considered an illness, and it doesn’t usually impact your health.

But it can be problematic during pregnancy.

If you are Rh negative and the father of your baby is Rh positive, your baby might inherit the Rh-positive gene. Stanford Children’s Health explains that the Rh-positive gene is dominant. So even when it’s paired with an Rh-negative gene, the positive gene is dominant. If both parents are Rh negative, the baby will be Rh negative too.

Rh Factor Complications

Complications can occur when the mother’s Rh factor is negative and her baby’s Rh factor is positive.

According to the Mayo Clinic, small amounts of your blood can come into contact with your baby’s blood during delivery or other times during your pregnancy, including:

  • amniocentesis, a prenatal test that involves retrieving a sample of the amniotic fluid in your uterus
  • chorionic villus sampling, a prenatal test that involves retrieving a sample of the placenta
  • fetal blood sample
  • bleeding during your pregnancy
  • ectopic pregnancy, when the fertilized egg implants somewhere other than the uterus, often in one of the fallopian tubes

If both mother and baby are Rh positive, compatibility isn’t an issue. But if the mother is Rh negative and the baby is Rh positive, the mother’s body may respond with an allergic reaction to the blood of the baby.

The mother may produce proteins called Rh antibodies. According to the American Congress of Obstetrics and Gynecologists (ACOG), someone with Rh-negative blood who produces Rh antibodies is considered “Rh sensitized.”

This isn’t normally a concern with the first pregnancy, because in many instances the baby is born before the mother’s body can develop a significant amount of antibodies. But without treatment during the first pregnancy, it can be an issue with any pregnancies that follow.

If your next baby is also Rh positive, your body will produce the Rh antibodies again. This time, they can cross the placenta and attack your baby’s red blood cells. Without enough red blood cells, your baby won’t get enough oxygen. This condition is known as hemolytic disease or hemolytic anemia. It can lead to illness, brain damage, and even death to your baby before or after delivery.

It’s very important to prevent your body from producing these antibodies. Otherwise, every Rh-positive baby you carry after the first pregnancy could be at risk for this condition.

Rh Factor Antibody Screen

Women who test negative for the Rh factor early in their pregnancy may need a second blood test, or antibody screen, in or around week 28.

The antibody screen tests for the presence of antibodies in Rh-positive blood. If no Rh antibodies can be detected, an injection of Rh immunoglobulin (Rhlg) will be administered. This prevents a woman’s body from producing Rh antibodies during her pregnancy.

If you test positive for the presence of Rh antibodies, the injection won’t be effective. Instead, your baby will be closely monitored throughout your pregnancy.

In severe cases of anemia, ACOG cautions that it may be necessary to delivery your baby before 37 weeks. A blood transfusion may be necessary immediately after delivery or through the umbilical cord while the baby is still in your uterus. If anemia is present but mild, it may be possible to deliver your baby at full term.

If a baby is born with Rh-positive blood, the mother will receive another injection of Rhlg within 72 hours after delivery.

If you or your baby’s father test positive for the Rh factor, it’s important to take certain precautions during your pregnancy. Contact your doctor immediately if you experience vaginal bleeding.

The Mayo Clinic recommends scheduling an Rhlg injection with your doctor. It’s also important to inform the doctors and nurses assisting you during labor and delivery if you and your baby are not Rh compatible.