What blood type will my child have?

How inheritance works

Each parent passes on one of their two ABO alleles: group A can pass A or O, group B passes B or O, AB passes A or B, and O only passes O. A and B are dominant over O. For the Rh factor, + is dominant: two Rh− parents can only have Rh− children, while two Rh+ parents can also have Rh− children.

The surprising case

An AB parent and an O parent can have neither AB nor O children: only A or B. And two group-A parents can absolutely have an O child, if both carry the silent allele.

Why we say possible and not probable

This is the most important choice in this tool, and it's a choice about honesty. From the parents' groups you can say with certainty which groups are possible and which are excluded, but not how often they'll come up, and the reason is that the visible group doesn't reveal the full genetic make-up: a person of group A can be AA or A0, and the two give different probabilities to the children. Since they can't be told apart from outside, any percentage would be invented.

For the same reason counting the children already born doesn't help either: every child is an independent draw, exactly like the sides of a coin.

The Rh factor, and why it matters in pregnancy

Rh positive is dominant, so two Rh negative parents can have only Rh negative children, while two Rh positives can also have a negative child if both carry the silent allele. So far it's genetics; the part that concerns health is another matter.

If the mother is Rh negative and the foetus is Rh positive, the mother's body can produce antibodies against the baby's red cells: it isn't a problem for the first pregnancy, but it becomes one for later ones. It's a well-known situation, prevented with an injection of immunoglobulin at the right moment, and that's why blood group is one of the first tests done in pregnancy. It's also why this page isn't a toy: knowing your own Rh is useful.

The exceptions that really exist

The rules on this page cover the vast majority of cases, but biology has a few documented rare ones: the Bombay phenotype, in which someone who would genetically be A or B tests as 0; some weak subgroups of A and B that are sometimes read as 0; and cases where a person carries two different genetic make-ups in the same body. They're rarities, but they exist, and that's why blood group is not proof of paternity: it can exclude, it can't confirm, and only a DNA test gives certainty.

Nearby tools

For pregnancy dates there are Conception date and Ovulation calculator. For an estimate of a child's adult height there is Child's future height, and for growth curves Growth percentiles.