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

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

Bllod Types 102

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. The geneticists at Stanford wrote a great explanation to this on the website for The Tech Museum of Innovation.

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.

Dr. Alan Greene

Dr. Greene is the founder of DrGreene.com (cited by the AMA as “the pioneer physician Web site”), a practicing pediatrician, father of four, & author of Raising Baby Green & Feeding Baby Green. He appears frequently in the media including such venues as the The New York Times, the TODAY Show, Good Morning America, & the Dr. Oz Show.

  1. meena

    hi, my blood group is B – and my husband is AB+ what precaution we have to take before pregnancy????

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  2. Shay

    Hello I’m O+ and my baby is B+ and her father is B+ is that a good way to be certain that he’s her Father?

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  3. vicky shaw

    sir,
    I am ab- negative and my wife is o+ positive blood group ,so our born baby have any problem?

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  4. Lynnrose

    My type O+ and my husband AB+ and I have sex only with my husband and my son is O+ and my daughter O+ so are you saying its impossible…

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    • Perhaps you and your husband should have your blood type tested again. It’s amazing how many people think they know their blood type and find out it’s different than what the believed it to be.

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  5. theresa

    hi, am A+ and slept with two men… one is O+ and the other is A+.
    My child came out to be O+, would like to know who have the highest possibility as her father and how many percent accuracy would that be?
    I understand our parents (child’s grandparents) blood type would also affect my child’s blood type – if so how could I determine without going for paternity test?

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    • If the mother is A+, here are the possible outcomes:

      – If the father is O+, the baby has a 75% chance of being type A and a 25% chance of being type O and a 93.75% chance of being Rh+ and 6.25% chance of being Rh-

      – If the father is A+, the baby has a 93.75% chance of being type A and a 6.25% chance of being type O and a 93.75% chance of being Rh+ and 6.25% chance of being Rh-

      In other words, it is not possible to know the father of the baby without a paternity test.

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  6. michelle

    Halo. If I, the mother has a bloodtipe o- and slept with 2 diffrent men the ones bloodtipe is o+ and the other is a+, which blood tipe would it possibly be, so that I would know which of the 2 is the father of my baby? So confused at this stage.

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    • We know you are O- (and I assume you are really sure of this):

        — If Dad is O+ the baby will be type O and can only be type O. 75% chance of being + and 25% chance of being -.

        — If Dad is A+ the baby has a 75% chance of being type A and a 25% chance of being type O and a 75% chance of being + and 25% chance of being -.

      What this means:

        — If the baby is type O, it could be either father.
        — If the baby is type A, the type A man is the father.
        — The Rh (+ / -) does not help rule out either man.

      If the baby has type O blood, DNA paternity testing can help you determine which man is the father.

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  7. Victoria

    My son is blood type o positive and my boyfriend is o negative. What are the chances that he is my son’s father?

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    • It is possible if your blood type is A+, B+, or O+

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  8. Samantha

    My mom is A positive and my dad is O positive. My sister is O positive too, however I am AB positive. Looking at multiple blood type chart it consistently indicates that an AB positive child is impossible. Is that true or is it possible under certain circumstances?

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  9. Anonymous

    Dear Doctor, my father’s blood type is AB and mine is O, is it possible that he is my father? I wonder if you can send me an e-mail to this answer?

    Regards!

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  10. Cindy

    The father is AB+ and the mother is O+ and they have a son who is O+. These parents are both white. The mother had no other sex partners. Are you saying it is impossible for them to have an 0+ child?

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    • Arenae

      I am O+. My dad is type AB and my mom is O. Research “Bombay” blood type or hh blood type. It will give you more explanation as to why this is.

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  11. Archie

    My mom is B+ ve and dad is AB. I’m O positive.. Is it possible?

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    • I’m assuming you’ve looked at the chart on this page? If so, your real question may be, “what do I do now?” There are several options. I recommend that you talk to your mom first and tell her your fears. If she is firm that your mom and your dad are your biological mother and father, then it’s pretty easy to get everyone retested to make sure the blood types are what you think they are.

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  12. David Nandwa

    My blood group is o positive and my wife is b negative. What precautions can we undertake to our unborn baby. She is 6 months pregnancy.

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  13. Emmanuel Yeboah Dunga

    if both parents are blood group O, then, under normal circumstances, all the children should have blood group O and no other group. But you can do the testing again if there is any doubt

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  14. Lori T.

    Hi my mom said she is AB- and my dad is B+. I’m O-. I know my mom has the RH negative factor, which I have as well. Is it possible my dad is my dad?

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  15. Susan Knight

    My parents are both A positve…I am B negative…is this possible?

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    • There is a small chance that two A+ parents could have a child who was Rh-, but two parents with type A blood can’t have a child with blood type B.

      Before you jump to conclusions, it’s wise to have all three of you tested again to confirm your blood types. It’s not unusual for people to think they are one type when they are another.

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  16. Reynaldo

    me and my wife are type O but my eldest son blood type is type AB. Is it Possible?

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    • Tina

      No, sorry to have to inform you, but its not possible. Maybe you all should be retested.

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  17. Namrata

    My Blood Group Is A+ and My Husband blood group is O- Please let us know which blood group or children will have? Also please let us know what are the precaution we have to take during the pregnancy? And is there any problem in making baby if blood group is A+ and O-

    Thanks & Regards,

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  18. Martin Luther

    My Blood Group Is B+ and My Wife is B- Please let us know which blood group or children will have? Also please let us know what are the precaution we have to take during the pregnancy?

    Thanks & Regards,
    Martin Luther

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  19. vinit

    My blood group is B positive & my wife has B negative…what is the possibilities of having baby…with blood group..can we have only one baby…or more???? Reply fast plzzz. It’s urgent! ***plzzz

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  20. Nikhil

    My blood group is B positive and my wife blood group is O positive; Is it possible that my baby have blood group O negative.

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  21. Michelle

    Hi Dr, both my parents are O and I am A+. My mom had lots of blood transfusions before I was born. Is this possible?

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