About the Blood Type Calculator
Blood Type Calculator shows which blood types a child can have when you know the blood types of both parents. Choose the ABO group (A, B, AB or O) and the Rh factor (positive or negative) of each parent and the page lists every possible blood type for the baby and the types that are not possible. It is based on how the ABO and Rh genes are inherited: A and B are dominant over O, and Rh positive is dominant over Rh negative. It tells you what is possible, not how likely each result is.
Updated October 2026
How to use the Blood Type Calculator
- 1Choose the blood group of parent 1 and parent 2.
- 2Choose the Rh factor (positive or negative) of each parent.
- 3Read the list of possible blood types for the child.
- 4See which types are not possible.
- 5Copy the result.
The ABO rules in plain words
You get one ABO gene from each parent. The gene can be A, B or O. If you have A and O, your group is A, because A hides O. If you have B and O, your group is B. A and B together give AB, and two O genes give O. That means a parent with group A or B may secretly carry an O gene and pass it on.
Reading the result
The calculator goes through every pair of genes that fits each parent's group and combines them, so the list covers all possibilities. If both parents are A, the child may be A or O. If one parent is AB and the other O, the child can be A or B but not AB or O.
- O and O: only O
- A and B: any of A, B, AB or O
- AB and O: A or B
- AB and AB: A, B or AB
Rh factor and pregnancy
Rh positive (+) is dominant over Rh negative (−), so two negative parents always have a negative child, while a positive parent can pass on either. When a mother is negative and her baby positive, doctors give her an injection to protect later pregnancies. Your midwife will test for it.
Related tools
Estimate eye colour with the Baby Eye Color Calculator, find the due date with the Pregnancy Due Date Calculator and the fertile days with the Ovulation Calculator. The Punnett Square Calculator shows other traits.
Features
- All ABO groups and both Rh types
- Lists possible and impossible blood types
- Based on standard genetics
- Instant result
- Simple explanation of the rules
- Private: nothing is uploaded
Frequently asked questions
How is blood type inherited?
Each parent passes one ABO gene (A, B or O) to the child. A and B are dominant over O, and A and B together make AB. The Rh factor is passed in a similar way, with positive dominant over negative.
Can two O parents have an A child?
No. Parents with blood group O carry only O genes, so their children are O.
Can parents with A and B have an O child?
Yes, if both carry a hidden O gene. They can also have A, B or AB children, so all four groups are possible.
Can two Rh negative parents have an Rh positive child?
No, not in normal genetics. Two negative parents can only have a negative child.
Why can't it say how likely each type is?
Likelihood depends on the parents' exact genes. A parent with group A may be AA or AO, which a blood group alone does not tell you.
Does a surprising result mean something is wrong?
Not necessarily. Rare conditions exist, such as the Bombay phenotype, and lab errors happen. A doctor or genetic counsellor can advise.
Is Rh incompatibility a risk?
If a mother is Rh negative and her baby is Rh positive, her body can make antibodies. Doctors prevent this with an injection. Ask your doctor or midwife.
Is this medical advice?
No, it is general information. Only a blood test gives a medical answer.