Blood type calculator for parents and baby

Your blood type has two parts, each inherited separately:

• ABO group (A, B, AB or O), set by one gene with three common versions (alleles): A, B and O. A and B are both dominant over O, and A and B together give AB. Every person carries two alleles, one from each parent, so someone with type A is either AA or AO, and type B is BB or BO.
• Rh factor (positive or negative), set by the RhD gene. D (positive) is dominant over d (negative), so an Rh-positive person is DD or Dd and an Rh-negative person is dd.

Each parent passes one of their two alleles to the child, at random. The calculator works through every combination, like a Punnett square, for both genes.

For Rh: two Rh-negative parents have an Rh-negative child. If one or both parents are Rh-positive, the child can be either, because a positive parent may carry a hidden d.

When you only enter blood types, the calculator doesn't know whether a type A parent is AA or AO. It assumes each possible genotype is equally likely and averages the results. That's why the percentages are estimates, and the real chances depend on your families.

Example with the default A+ and B+ parents: AB about 56%, A about 19%, B about 19%, O about 6%, and Rh-negative about 6%.

If you know a genotype, check I know the genotype too. A clue: if a type A parent has a type O parent (the child's grandparent), they must be AO. If an Rh-positive parent has an Rh-negative parent, they must be Dd. Entering genotypes makes the percentages exact under standard inheritance.

Switch to What could the other parent be? and enter one parent's type and the child's type. The tool lists the ABO groups and Rh factors the other parent could have under standard inheritance, and which ones are ruled out. If no type fits, it says so: for example, a type AB parent can't have a type O child, whatever the other parent's type.

Standard rules have rare exceptions: the cis-AB variant, the Bombay phenotype, weak or partial D, chimerism, and lab errors. So an "impossible" result is very unlikely, not literally impossible, and blood type can't establish or rule out paternity; that requires a DNA test.

If you're pregnant and Rh-negative, your prenatal care will include checking for Rh incompatibility, which can be managed with Rh immune globulin. Talk to your doctor or midwife about it.

This calculator is for information only and is not medical advice. It runs entirely in your browser. For another inherited trait, try the eye color calculator.