Baby Blood Type Calculator

Which blood types the children of two parents can have and which are ruled out, in the ABO system and the Rh factor. It gives possible and impossible, not percentages, and explains why.

Blood types the children can have

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Blood types the children can have—

Types ruled out—

ABO system—

Rh factor—

Father's possible genotypes—

Mother's possible genotypes—

How this was worked out

The formula

Father's alleles × mother's alleles

What it means

ABO blood type is decided by three alleles: A and B are codominant with each other and both dominate O. That is why a type A person can be "AA" or "AO" and carry a hidden O, while a type O person can only be "OO". The Rh factor is separate: positive dominates negative, so two positive parents can have a negative child if both carry the allele, but two negative parents cannot have a positive one.

How to work it out by hand

Write down the genotypes each parent could have, given their blood type
Cross every allele from the father with every allele from the mother
Translate each pair of alleles into the type you see: AO is A, and only OO is O
Repeat the cross for Rh, where positive dominates negative

What is worth knowing

You will not see percentages, and that is deliberate: giving a probability would need to know whether the type A parent is "AA" or "AO", and the blood type does not say. Any figure would come from assuming how common each allele is in one particular population, which changes from country to country. This is also not a paternity test: it can rule out, never confirm, and there are documented exceptions such as the Bombay phenotype and chimerism. For an answer with legal or medical weight you need a laboratory test.

Frequently asked questions

Can two type O parents have a type A child?

No. Type O carries only O alleles, so every child is type O. It is the clearest exclusion in the system.

Can two Rh negative parents have an Rh positive child?

No, apart from extremely rare exceptions. Positive is dominant: if neither parent carries it, there is nowhere for it to come from.

Why no percentages?

Because they cannot be worked out without knowing the genotype. A type A parent may be "AA" or "AO", and assuming half and half would mean inventing a population statistic.

Does this work as a paternity test?

No. It can exclude in some cases but never confirm: millions of people share a blood type. A real test looks at DNA markers.

Can an AB parent have a type O child?

Not in the ordinary case: an AB parent always passes an A or a B. The documented exception is the Bombay phenotype, which is very rare.