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?

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 use blood tests to exclude 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. Here is a list of possible and impossible situations:

Parents’ Blood Types

Possible Children

Impossible Children

A & A

A, O


A & B

A, B, AB, O


A & AB

A, B, AB


A & O

A, O


B & B

B, O


B & AB

A, B, AB


B & O

B, O



A, B, AB


AB & O

A, B


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, at

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.

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.

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 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 (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.

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  1. 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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