Unlike most pregnancy guessing games, this one has a real answer. Blood type follows well understood rules of inheritance, so a baby blood type calculator can tell you exactly which types your child can have, and just as importantly, which ones are flat out impossible.

Pick both parents’ blood types below and you will see all eight possibilities, with the impossible ones ruled out.

Why people use this tool

• It gives a definitive answer, not a guess. The list of blood types your baby can inherit is genetically exact. This is not a coin flip like the Chinese calendar or a wedding ring test.
• It flags a real medical issue. If the mother is Rh negative and the baby could be Rh positive, the tool says so. That combination genuinely matters in pregnancy, and it is worth raising with your doctor.
• It catches impossible combinations. Add the grandparents and the calculator will tell you if the blood types you entered cannot fit together genetically.

How ABO blood type inheritance works

Your ABO blood type comes down to two genes, one from each parent. There are three versions of that gene, called alleles: A, B, and O.

A and B are both dominant over O, but neither is dominant over the other. That gives four possible blood types from three alleles:

• Type A means you carry either AA or AO
• Type B means you carry either BB or BO
• Type AB means you carry AB, one of each
• Type O means you carry OO, the only combination that produces type O

Here is the part most people miss. If someone is type A, you cannot tell by looking whether they are AA or AO. That hidden O allele is invisible, and it can still be passed to a child. It is exactly why a type A parent and a type B parent can, surprisingly, have a type O baby.

Blood type chart: what your baby can inherit

This chart covers every ABO combination. Rh is inherited separately, so it gets its own section below.

Two rows are worth pausing on.

O + AB is the strange one. These parents cannot produce a child who matches either of them. The AB parent always passes an A or a B, and the O parent always passes an O, so every child comes out AO or BO, which shows up as type A or type B. Never AB, never O.

A + B is the wide open one. Two parents like this can produce any of the four blood types, because each of them may be hiding an O allele.

The Rh factor, and why Rh negative mothers need to know

The plus or minus after your blood type is the Rh factor, and it behaves as a straightforward dominant trait. Rh positive beats Rh negative, so:

• Two Rh positive parents can still have an Rh negative baby, if both are quietly carrying a negative allele.
• An Rh positive and an Rh negative parent can have either.
• Two Rh negative parents can only have an Rh negative baby. There is no hidden positive allele to pass on.

The combination that matters clinically is an Rh negative mother carrying an Rh positive baby. Their blood is not normally supposed to mix, but if it does, during delivery, a fall, or certain procedures, the mother’s immune system can start producing antibodies against Rh positive blood.

This rarely causes trouble in a first pregnancy. The concern is a later one, where those antibodies can cross the placenta and attack the blood cells of an Rh positive baby.

The reassuring part is that this is one of the best managed problems in modern obstetrics. Doctors give an injection of Rh immunoglobulin, often called anti-D or RhoGAM, usually around 28 weeks and again shortly after birth. It stops the antibodies from forming in the first place. Our calculator flags the situation for you, but your midwife or obstetrician will already be screening for it with a routine blood test.

Why adding the grandparents sharpens the answer

This is where the calculator does something most others do not.

Remember that a type A parent might be AA or AO, and their blood type alone will never tell you which. With nothing else to go on, the tool assumes both are equally likely. The grandparents can settle it.

Say the mother is type A and both of her parents are type AB. An AB person only ever passes an A or a B, so the mother could not have received an O from either of them. She must be AA, with no hidden O allele to give.

Now pair her with a type O father. Without the grandparents, a type O baby looks possible. With them, a type O baby becomes genetically impossible. The grandparents did not just shift the odds, they eliminated an entire outcome.

Open the optional grandparents section, fill in both people on either side, and the calculator works out that parent’s exact genes and shows them to you. If the blood types you enter cannot possibly fit together, it will tell you that too.

What this calculator cannot tell you

Two honest limits, and the first one matters a great deal.

This is not a paternity test. Blood types can occasionally rule a man out, but they can never confirm one, because millions of people share any given blood type. Any conclusion about paternity drawn from a blood type chart is unreliable, and the stakes are far too high to gamble on it. Only DNA testing can answer that question.

Rare exceptions to the rules exist. A small number of people have unusual genetics, such as the Bombay phenotype, where a person tests as type O yet actually carries an A or B allele they can still pass on. These cases are uncommon, but they are real, and they mean no chart is completely airtight.

For your baby’s actual blood type, the answer comes from a simple blood test after birth. This calculator is built for understanding and curiosity, not diagnosis, and it does not replace advice from your doctor or midwife.

Tracking other milestones? Our Due Date Calculator estimates when your baby will arrive, and the Pregnancy Week Calculator tells you exactly where you are right now. For another look at what your baby might inherit, try the Baby Eye Color Calculator, though that one is far less predictable than blood type. And if you are in the mood for a purely for fun prediction, the Chinese Gender Predictor is waiting.