What Blood Type Will My Baby Have? A Genetic Explanation

Updated: February 23, 2021
There's no way to be sure what your baby's blood type is without a blood test, but you can narrow down the possibilities if you know your blood type and your partner's.
Blood Type

Wondering what your baby's blood type will be? Blood type is inherited from both biological parents. There's no way to be sure what your baby's blood type is without a blood test, but you can narrow down the possibilities if you know your blood type and your partner's.

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What is Blood Type?

Red blood cells have substances on their surfaces that vary from person to person. The particular substance on your own blood cells determines your blood type. There are two blood group classifications: the ABO system and the Rh system.

ABO Blood Group

The ABO system has four major blood groups: A, B, AB, and O. Red blood cells in blood type A have A antigens on their surfaces. Red blood cells in blood type B have B antigens on their surfaces. Red blood cells in blood type AB have both antigens, and those in blood type O have no antigen at all.

Inheritance and the ABO Gene

ABO blood type is inherited just like any other trait. Every person carries two genes, or alleles, for each trait. One ABO allele is inherited from the father (who passes on one of his two) and the other is inherited from the mother (who passes on one of her two).

The two alleles passed on to your baby will make up his genotype. Genotype is the genetic code that you can pass on to your children, but not all alleles in your genotype will show up as traits in you. The actual blood type your baby ends up with (his phenotype) depends on gene dominance. For example, a baby who gets a blood type B allele from one parent and a blood type O allele from the other will end up with blood type B because the B allele is dominant. Also, it is possible for a baby to have a different blood type than both parents.

Here is the full break down:

  • A is dominant over O
  • B is dominant over O
  • A and B are co-dominant

Therefore:

  • Two parents with blood type A will have a baby with either A or O
  • Two parents with B blood type will have a baby with either B or O
  • Two parents with blood type AB will have a baby with either A, B, or AB
  • Two parents with the O blood type will have a baby with O
  • One parent with blood type A and one parent with the B blood type might have a baby in any blood group
  • One parent with blood type A and one parent with blood type AB will have a baby in the A, B, or AB blood groups
  • One parent with blood type A and one parent with the O blood type will have a baby with A or O
  • One parent with the B blood type and one parent with blood type AB will have a baby in the A, B, or AB blood groups
  • One parent with the B blood type and one parent with the O blood type will have a baby with B or O
  • One parent with blood type AB and one parent with the O blood type will have a baby with either A or B

The Rh Factor

The Rhesus (Rh) Factor is a protein found on the surface of red blood cells. Positive blood has the Rhesus protein, while negative blood does not. For example, if your ABO blood group is AB and you are Rh positive, your full blood type is AB positive. If your ABO blood group is O and you are Rh negative, your blood type is O negative. There are eight possible blood types when you combine the ABO groups and the Rh groups: A+, A-, AB+, AB-, B+, B-, O+, and O-.

Inheritance and the Rh Factor

The Rh factor is inherited on the RHD gene and Rh positive gene is dominant. Even if both parents are Rh positive the baby can be Rh negative. This is because it's possible that both parents carry an Rh negative allele and they might each pass this gene to the baby. However, if both parents are Rh negative, you can be sure the baby will be too.

  • If both parents are Rh positive, the baby might be positive or negative
  • If one parent is Rh positive and the other parent is Rh negative the baby might be positive or negative
  • If both parents are Rh negative, the baby will be Rh negative

How the Rh Factor Affects Pregnancy

If you are Rh negative, it could impact your pregnancy if your baby is Rh positive. This is called Rh incompatibility. If your blood mixes with your baby's blood, there is a chance that your body won't recognize the Rh protein, and it will produce antibodies called Rh antibodies. Rh antibodies won't hurt your first pregnancy, but they might harm subsequent pregnancies. In your next pregnancy, these antibodies can cross the placenta and attack the baby's red blood cells, causing life threatening anemia. If you are Rh negative and your baby's father is also Rh negative, there is no need to worry about Rh antibodies. However, if your partner is Rh positive, your doctor will conduct screenings during pregnancy delivery to see if your body is producing antibodies. You may also need an injection called Rh immune globulin to stop your body from producing antibodies.

Why Does Blood Type Matter?

Knowing which ABO blood group your baby is in is important in case a blood transfusion is ever needed. Some blood types can mix with others while some cannot. For example, the AB+ blood type can receive blood from any other type and the O- blood type can give to any other blood type. In an emergency, when a person's blood type is unknown, O- blood is used. However, fresh blood for a transfusion is not always available so it's good to know which type you are.