Blood Types 102: The Role of A, B, O, and AB Groups In Determining Paternity

The use of blood type in determining paternity is beneficial by excluding some men from being the fathers of some children. It is not a definitive test.

Question

Hello Doc, Is there any way of determining the father of a child before it is born?
Jamaica

Dr. Greene's Answer

Having a baby is a lifelong responsibility and hopefully an even greater reward — for somebody! As a mother’s belly swells, she knows with deep certainty that the child is hers. Each time the baby moves or kicks, the bond between her and her child grows.

Depending on the situation, the father may be pretty sure that the child is his. For most of history, though, dads have had to rely on circumstantial evidence as the foundation on which to build this crucial relationship. After the baby was born, he could feel more sure he was the dad if the baby looked like him (“He has your feet, Honey!”), but often these early resemblances are at least partially creative imaginations.

In 1901 biologist Karl Landsteiner distinguished between three types of blood — groups A, B, and O. A fourth group — AB — was discovered a year later by another research team. As the inheritance patterns of these blood groups were worked out over the next decades, it became possible to include the use of blood type in determining paternity by excluding some men from being the fathers of some children. For instance, if the parents both have blood type O, then the children must all have blood type O. If a child were to have blood type A, B, or AB, then the presumed father must not be the real father. If the child’s blood type were O, then the presumed father might be the real father — but so might millions of other men.

Using Blood Type in Determining Paternity:Possible and Impossible Situations

Parents’ Blood Types

Possible Children

Impossible Children

A & A

A, O

B, AB

A & B

A, B, AB, O

none

A & AB

A, B, AB

O

A & O

A, O

B, AB

B & B

B, O

A, AB

B & AB

A, B, AB

O

B & O

B, O

A, AB

AB &AB

A, B, AB

O

AB & O

A, B

AB, O

O & O

O

A, B, AB

 

These are general rules, though, and exceptions apply. Very rarely, gene mutations may change the rules such that “impossible children” become possible.

Today there are over 600 blood types known (as well as other tissue types called HLA types), which can make paternity testing far more accurate — but still not perfect.

Determining Father Before a Baby is Born

It is also now possible to determine the father before a baby is born. This is done by comparing DNA molecules — our genetic blueprints. To do this you need a blood sample from both the mother and the potential father (testing without the mother’s blood is possible, but more difficult — and more expensive). You also need a small sample of amniotic fluid (the water that the baby is floating in). Less than 1/4 teaspoon is sufficient for the test. The amniotic fluid may be obtained by a process called amniocentesis. This procedure is performed no earlier than 13 weeks into the pregnancy.

A court order or informed consent of all adults involved is required to proceed with paternity testing.

You will need to wait 3 to 4 long weeks for the results. Waiting for these test results can be a very anxious time. Rush orders take 10 to 15 business days, but cost about $500 extra.

Either way, if the test says that a man is not the father, then legally and truly he is not (it can absolutely exclude some men as the father of a certain child). If the test says that he is the father, then he probably is — there is about a 99.8% chance that he is. DNA testing is now legally accepted as able to determine paternity.

There are about one million two hundred eighteen thousand five hundred males in Jamaica (as of 1992). A positive DNA paternity test could limit the potential fathers to only about 2,437 of them (plus 0.2% of the tourists). Only 2 out of 1000 men could possibly be the father. As you can see, a positive paternity test is good evidence, but not an ironclad guarantee.

Paternity Testing

Prenatal paternity testing can be arranged through a company called Genelex, located in Seattle, Washington. They are very helpful, and can be reached at 1.800.523.6487 or healthanddna.com. The test costs $700.

If you wait until after the baby is born, DNA testing can be arranged through most local blood banks (many of which use Genelex). The blood sample can be obtained at birth. Otherwise, the baby should be at least 2 months old, since a fair amount of blood is needed for the test. In my area, this option costs about $600.

I realize that the circumstances that prompt a person to undergo paternity testing are often difficult. I hope that whatever you want turns out to be true. Even more, I hope that whatever turns out to be true becomes something that you learn to want.

Last medical review on: August 12, 2009
About the Author
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Dr. Greene is a practicing physician, author, national and international TEDx speaker, and global health advocate. He is a graduate of Princeton University and University of California San Francisco.
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Recent Comments

Hi Deseree,

The whole Rh- / Rh+ issue can be confusing, so let’s start there. If a person is Rh+ it means they have the D Antigen — they are positive for the D Antigen. But that does not mean they must pass it on. Two parents that are Rh+ have a 6.25% chance of having a child that is Rh-. So, your parents could be Rh+ and all their children could be Rh-. That would be completely normal.

On the other hand, if neither parent is Rh+, that is neither parent is positive for the D Antigen, they can not possibly pass that on. All their children will be Rh-.

Now as to the B+ and A+ parent having an O- child, we’ve already determined the Rh- is perfectly normal. But so is the type O blood. Here’s why — Blood types are comprised of two alleles. The most common (by far) are A, B, and O.
— If a person has type A blood he or she has at least one A allele. He or she can have two A alleles (AA) but can also have one A and one O (AO). Both are called type A blood.
— If a person has type B+ blood he or she has at least one B allele. He or she can have two B alleles (BB) but can also have one B and one O (BO). Both are called type B blood.

So if one of your sister’s parents had type B+ blood and the other had type A- blood there would be a 6.25% chance that she would have type O blood and a 25% chance she would have Rh- blood.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

I have a question regarding the RH my Father is B+ and my Mother is A+ but I am A- and my late sister was O- how does that work? I always thought that two people with RH+ could not produce a RH- child. on the other hand my husband and I are both A- and so are all three of our Children. I’m told this too is unusual.

Hi Deseree,

The whole Rh- / Rh+ issue can be confusing, so let’s start there. If a person is Rh+ it means they have the D Antigen — they are positive for the D Antigen. But that does not mean they must pass it on. Two parents that are Rh+ have a 6.25% chance of having a child that is Rh-. So, your parents could be Rh+ and all their children could be Rh-. That would be completely normal.

On the other hand, if neither parent is Rh+, that is neither parent is positive for the D Antigen, they can not possibly pass that on. All their children will be Rh-.

Now as to the B+ and A+ parent having an O- child, we’ve already determined the Rh- is perfectly normal. But so is the type O blood. Here’s why — Blood types are comprised of two alleles. The most common (by far) are A, B, and O.
— If a person has type A blood he or she has at least one A allele. He or she can have two A alleles (AA) but can also have one A and one O (AO). Both are called type A blood.
— If a person has type B+ blood he or she has at least one B allele. He or she can have two B alleles (BB) but can also have one B and one O (BO). Both are called type B blood.

So if one of your sister’s parents had type B+ blood and the other had type A- blood there would be a 6.25% chance that she would have type O blood and a 25% chance she would have Rh- blood.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Felix,

Thanks for writing in.

If one parent’s blood type is O and if the other parent has type A, they can only have children with type O or type A blood.

It’s not uncommon for blood types to be different than they are thought. It is wise to have all your blood retested.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Thanks for the knowledge, but if I may ask, My father’s blood type is O+ while my mother’s blood type is A, and mine blood type is B what could be the problem then?

Hi Felix,

Thanks for writing in.

If one parent’s blood type is O and if the other parent has type A, they can only have children with type O or type A blood.

It’s not uncommon for blood types to be different than they are thought. It is wise to have all your blood retested.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Harmony,

You mention that you have done this test twice. Have both your parents repeated the test to confirm their blood types? If so, and if your father has type O+ blood, and your mother has type B+ blood, it’s time for a paternity test.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hello Rohit,

The answer to your question is — yes! If one parent has type A+ blood and the other parents has type AB+ blood, they can have a child with type AB- blood. In fact, the odds are 37.5% that any of their children will have type AB blood and 6.25% that any of their children will be Rh-. If you’d like further explanation, please let me know.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

My Blood type is A+ and my wife blood type is AB+. Can the blood type of my child be AB-

Hello Rohit,

The answer to your question is — yes! If one parent has type A+ blood and the other parents has type AB+ blood, they can have a child with type AB- blood. In fact, the odds are 37.5% that any of their children will have type AB blood and 6.25% that any of their children will be Rh-. If you’d like further explanation, please let me know.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

I am AB+ blood group but my dad is O+ and my mum is B+. Is my dad my biological father. I have done this test twice and it is still AB+. My siblings are all O+.

Hi Harmony,

You mention that you have done this test twice. Have both your parents repeated the test to confirm their blood types? If so, and if your father has type O+ blood, and your mother has type B+ blood, it’s time for a paternity test.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

I’M O+ MY HUSBAND IS A+ OUR BABY IS O+ IS THIS POSSIBLE

Hi Rosamaria,

Yes! If one parent has type O+ blood and the other parent has type A+ positive blood, they can have a baby with O+ blood. In fact each of their children could be O+, O-, A+ or A-. All are normal outcomes.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Rosamaria,

Yes! If one parent has type O+ blood and the other parent has type A+ positive blood, they can have a baby with O+ blood. In fact each of their children could be O+, O-, A+ or A-. All are normal outcomes.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.