Baby blood type calculator
Pick both parents' ABO groups and rhesus factors. This shows every blood type the baby could have, which are impossible, and how likely each one is. It handles the rare exceptions honestly, and explains why blood type is never used to establish who a parent is.
Baby blood type calculator
No sign-up · PrivateThe letter part — A, B, AB or O. Group O is the only one whose underlying genotype is certain from the group alone.
The + or − after the letter. Inherited separately from the letter, so leaving it as “I don’t know” still gives you the ABO answer.
Blood groups get mis-remembered surprisingly often. A home or workplace test is not a medical record.
An RhD positive parent may carry one copy of the D gene or two, so a positive parent does not automatically mean a positive baby.
Blood types this baby could have
Possible, not predicted. Every one of these 4 is consistent with the groups you entered. Nothing here says which one your baby has.
Possible ABO groups: O or A — both are possible, and nothing here narrows it further.
ABO groups ruled out: B and AB — neither parent has the gene to contribute. Exclusions like these are the strongest statement this system makes, and even they have rare exceptions (see the notes).
If the genotypes are AA × OO: A — 100%. One of 2 genotype combinations consistent with the groups you entered — the percentages are conditional on this one being the case.
If the genotypes are AO × OO: O — 50%, A — 50%. One of 2 genotype combinations consistent with the groups you entered — the percentages are conditional on this one being the case.
Possible rhesus factors: RhD positive or RhD negative.
If the RhD genotypes are ++ × --: RhD positive — 100%.
If the RhD genotypes are +- × --: RhD positive — 50%, RhD negative — 50%.
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow blood type inheritance is worked out
Two independent systems are at work, and mixing them up is the usual source of confusion. ABO group and rhesus factor are inherited separately, so the tool solves them separately and then combines the answers.
Step 1 — the ABO group
You carry two ABO genes, one from each parent. There are three versions: A, B and
O. A and B are both dominant over O, and neither is dominant over the other — they are
codominant, which is why AB exists as a group in its own right.
That produces four groups from six possible gene pairs:
- Group A —
AAorAO - Group B —
BBorBO - Group AB —
ABonly - Group O —
OOonly
The consequence people find surprising: a parent with group A may be carrying a hidden O. Two
group A parents who are both AO have a one in four chance of a group O child. Group O is the only
group whose genotype is certain from the group alone.
Step 2 — combining the parents
The tool takes every genotype consistent with each parent's stated group, crosses them, and collects the results. Where a parent's genotype is uncertain the possibilities are listed rather than assumed, and the probabilities shown are conditional on the genotype combination, not a single flat percentage.
Some combinations rule things out cleanly. Two group O parents can only have a group O child, because there is
no A or B gene available to pass on. A group AB parent cannot normally have a group O child, because they have no
O to give. Those exclusions are the only part of this that is close to firm.
Step 3 — the rhesus factor
RhD positive is dominant over RhD negative. A positive parent may be ++ or +−;
a negative parent is −−. So two RhD negative parents have RhD negative children, while
two positive parents can have a negative child if both carry the negative version. This is why an RhD negative
baby can appear in a family with no obvious history of it.
Step 4 — the exceptions this tool flags rather than hides
Inheritance is not quite as tidy as a school textbook grid. Rare variants exist — the Bombay phenotype, weak and partial D types, cis-AB, and chimerism — which can produce results that look impossible on the grid above. They are genuinely rare, but they are real, and they are the reason this tool is presented as a genetics explainer rather than an answer about anybody's family.
Blood type must never be used to work out whether someone is a parent. It can occasionally exclude, it can never confirm, and the rare exceptions mean even an apparent exclusion can be wrong. Only DNA testing addresses that question.
What blood type will my baby have?
Blood type inheritance is one of the few pieces of genetics simple enough to work out on paper, which is why it turns up in every school biology course. The tidy version is mostly right. The places where it stops being tidy are worth knowing, because they are where people get into trouble with it.
Two systems, inherited separately
When someone says they are O positive, they are giving you two facts from two independent systems. The letter is the ABO group. The positive or negative is the rhesus D factor. They are inherited independently of each other, so you work them out separately and then put them back together at the end.
There are eight common combinations in total: A, B, AB and O, each either positive or negative. They are not evenly distributed — O positive is much more common than AB negative — and the distribution differs substantially between populations, which matters for blood donation far more than it does for predicting a baby.
The ABO part
You have two ABO genes, one inherited from each parent, drawn from three versions: A, B and O. A and B are each dominant over O, so a single A gene paired with an O gene produces group A. But A and B are codominant with each other — neither wins — so inheriting one of each produces group AB, a group with both antigens present.
This is why blood group does not reveal the full genetic picture. A person in group A is either AA or AO, and from the outside these look identical. The O is carried silently and can be passed on. Group O is the exception: because O is recessive, being group O means having two O genes, with nothing hidden.
Where that leads
Two group A parents can have a group O child, and often assume something has gone wrong when they do. It has not. If both parents are AO, each has a one in two chance of passing on the O, so one child in four will be OO and therefore group O. The same logic applies to two group B parents.
Some things genuinely are impossible in the standard model. Two group O parents cannot produce a child in group A, B or AB, because neither parent has an A or a B gene to contribute. A group AB parent cannot normally produce a group O child, because they carry no O. These exclusions are the strongest statements the system makes, and even they have rare exceptions.
The rhesus factor
RhD positive is dominant. If you are RhD negative, you inherited a negative version from both parents, meaning both parents carry at least one negative version even if both are themselves positive. That is how two positive parents produce a negative baby, which surprises people rather often.
Rhesus status carries real clinical weight in pregnancy, unlike the ABO group. If a woman is RhD negative and her baby is RhD positive, some of the baby's blood entering her circulation can prompt her immune system to produce antibodies against RhD. This rarely affects a first pregnancy, but those antibodies can cross the placenta in a later pregnancy and attack a RhD positive baby's red blood cells — rhesus disease, also called haemolytic disease of the fetus and newborn.
This is a solved problem in countries with routine antenatal care. Blood group and antibody screening happen early in pregnancy, and anti-D immunoglobulin is offered to RhD negative women to prevent the antibodies forming. Some services now test the baby's RhD genotype from the mother's blood first, so that anti-D is given only where the baby is actually RhD positive. If you are RhD negative, this is a conversation to have with your midwife, and this site has a separate tool covering what happens where you live.
Where the neat grid breaks down
Textbook inheritance covers the overwhelming majority of people, and then there are the exceptions. The Bombay phenotype involves a separate gene needed to build the A and B antigens at all; without it, a person tests as group O regardless of what ABO genes they carry, and can pass on an A or B their own blood type does not show. Weak D and partial D variants sit between positive and negative and are typed differently by different laboratories. Rarer still, cis-AB passes A and B together on one chromosome, and chimerism means carrying two cell populations with different types.
None of these are common. All of them are documented. Together they mean that an apparently impossible result is far more likely to be a rare variant or a clerical error than the thing people fear it is.
Blood type is not a paternity test
This deserves saying without hedging. Blood groups can sometimes exclude a possible parent, they can never confirm one, and the rare variants above mean that even an exclusion can be mistaken. Enormous distress has been caused by people running the school grid on their own family and drawing a conclusion from it. If that is a genuine question, DNA testing is the only thing that answers it, and this calculator should be closed rather than consulted.
How you find out for certain
The only way to know a baby's blood type is to test it. It is not routinely tested at birth in healthy babies in the UK, because for most families the answer changes nothing. It is tested when there is a clinical reason — if the mother is RhD negative, if there are concerns about jaundice, or if the baby needs a transfusion. So if you finish this calculator with three possible answers and no way to narrow it down, that is the expected outcome rather than a failure of the tool.
Sources
- Blood types — NHS Blood and Transplant, accessed
- Blood groups — NHS, accessed
- Rhesus disease — NHS, accessed
- The Rh factor: how it can affect your pregnancy — American College of Obstetricians and Gynecologists, accessed
- High-throughput non-invasive prenatal testing for fetal RHD genotype (DG25 / HTG420) — NICE, accessed