What blood type will they have?

Pick both parents' ABO type and Rh factor. We'll compute the probability of every possible outcome for your baby — using real Mendelian genetics, not vibes.

How ABO and Rh inheritance actually works

This calculator takes both parents' ABO blood type and Rh factor and computes the probability of every possible outcome for the baby, using Mendelian genetics rather than a guess. ABO and Rh are both well-understood, single-locus inheritance systems, so when the parents' genotypes are fully known, the math behind the prediction is exact.

ABO blood groups come down to three alleles — A, B, and O. A and B are codominant with each other, and both are dominant over O. That means a person who is blood type A could be genotype AA or genotype AO, and the same goes for type B (BB or BO); type AB and type O only have one possible genotype each. The catch: phenotype alone (what your blood test says) doesn't reveal which genotype an A or B parent actually has. We handle that with US population priors — roughly 26% A allele, 6% B allele, and 67% O allele — to weight the probability of a hidden O allele, unless you already know (for example, because that parent has had an O-type child before), in which case toggling it makes the prediction exact instead of probabilistic.

Rh factor works the same Mendelian way but with a single D gene: Rh-positive is dominant over Rh-negative. Combine the two systems and there are eight possible outcomes for the baby (A, B, AB, or O, each either Rh-positive or Rh-negative), and the calculator's full breakdown shows a probability for each. One consequence of the hidden-allele problem worth knowing: two parents who are both blood type A can still have an O-type baby, if both happen to carry a hidden O allele. Two O-type parents, on the other hand, can only ever have an O-type child, since O is a double-recessive genotype with nothing else to pass on.

Rh factor also matters for the pregnancy itself, not just the prediction. If mom is Rh-negative and dad is Rh-positive, there's a real chance the baby is Rh-positive, and mom's body can form antibodies against the baby's blood — during this pregnancy or a future one. The standard, well-studied response is RhoGAM (anti-D immunoglobulin), given around 28 weeks and again within 72 hours of delivery; it's routine and highly effective, and your OB will already be watching for it if your Rh factors don't match.

Limitations worth being honest about: this tool estimates probabilities, not certainties, whenever a parent's exact genotype isn't known. It also doesn't account for rare ABO subgroups (cis-AB, the Bombay phenotype), incorrect assumed paternity, or chimerism — all real, but all uncommon.

Sources: Sanger 1968 on ABO; Race & Sanger 1975 (Blood Groups in Man); AABB Technical Manual; ACOG Practice Bulletin 192 for Rh management.