How is a baby's blood type determined?

A baby inherits one ABO allele (Iᴬ, Iᴮ, or i) and one Rh allele (D or d) from each parent. Iᴬ and Iᴮ are codominant and both dominant over i, so IᴬIᴬ or Iᴬi is group A, IᴮIᴮ or Iᴮi is group B, IᴬIᴮ is AB, and ii is O. D is dominant, so DD or Dd is Rh-positive and only dd is Rh-negative.

Can two parents with blood type O have a type A or B baby?

Under standard inheritance, no. Type O parents are both ii, so every child is ii (type O). Very rare exceptions, such as the Bombay phenotype or a cis-AB allele, can make blood typing look inconsistent with the parents; these need specialist laboratory testing.

Can two Rh-positive parents have an Rh-negative baby?

Yes. If both parents are Rh-positive carriers (genotype Dd), each child has a 25% chance of inheriting d from both and being Rh-negative (dd). Two Rh-negative parents (dd × dd), by contrast, can only have Rh-negative children.

Why does the calculator show 'possible' instead of percentages?

A blood test reports your phenotype (for example A+), not your genotype. A type A parent could be IᴬIᴬ or Iᴬi, and the odds for the child differ between the two. Instead of guessing with population averages, the default mode shows which child blood types are possible for any matching genotype. Turn on the advanced toggle if you know the exact genotypes to get Punnett-square percentages.

What are the odds if one parent is Iᴬi and the other is Iᴮi?

The ABO Punnett square gives four equally likely outcomes: IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B), and ii (O), so 25% each. If both parents are also Dd for Rh, multiply by 75% Rh-positive or 25% Rh-negative: for example A+ is 25% × 75% = 18.75%.

Can blood type prove or rule out paternity?

No. Blood type can only show that a combination is unlikely under standard genetics, and rare variants break those rules. Many unrelated people share the same blood type. Only DNA testing can establish parentage.

What is Rh incompatibility in pregnancy?

If an Rh-negative mother carries an Rh-positive baby, her immune system can make anti-D antibodies that may affect this or a later pregnancy (haemolytic disease of the newborn). It is largely preventable with anti-D immunoglobulin. If you are Rh-negative and pregnant, discuss testing and prophylaxis with your clinician.

Health

Baby Blood Type Predictor

Predict your baby's blood type from both parents' ABO group and Rh factor. See which types are possible, or exact Punnett-square odds if you know the genotypes.

A = IᴬIᴬ | Iᴬi
B = IᴮIᴮ | Iᴮi
AB = IᴬIᴮ · O = ii
Rh+ = DD | Dd · Rh− = dd
P(child group) = P(ABO) × P(Rh) — the genes are inherited independently

Suggested by Mrs Aftab (MMG student)

Advanced: I know the parents' genotypesShowing which child blood types are possible. Turn on to pick exact genotypes (e.g. Iᴬi, Dd) and see percentages.

Parent 1 (blood group)

Parent 2 (blood group)

Child blood type

A+Possible
A−Possible
B+Possible
B−Possible
AB+Possible
AB−Possible
O+Possible
O−Possible

Prediction only, based on standard ABO and Rh inheritance. Rare variants (Bombay phenotype, cis-AB, weak or partial D) can produce results outside these predictions, so this is not a paternity test. If you are pregnant and Rh-negative, ask your clinician about Rh incompatibility and anti-D prophylaxis.

How to use the Baby Blood Type Predictor

Enter your valueType the number you want to work with into the Baby Blood Type Predictor input field.
Choose your units or optionsSelect the units or options that match your figure. Calculy handles the conversion or formula for you.
Read the resultThe result updates instantly — there is no button to press and no sign-up required.

What is a baby blood type predictor?

Your blood type is set by two genes you inherit from your parents: the ABO gene, which decides whether you are group A, B, AB, or O, and the RhD gene, which decides whether you are Rh-positive or Rh-negative. Each parent passes on one copy (allele) of each gene, chosen at random.

This calculator applies those Mendelian rules to both parents' blood types. By default it shows which of the eight child blood types (A+, A−, B+, B−, AB+, AB−, O+, O−) are possible and which are not possible. If you know the parents' exact genotypes, the advanced mode gives exact Punnett-square percentages.

How blood type inheritance works

The ABO gene has three alleles: Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant (both show when present together), and both are dominant over i. For Rh, the D allele is dominant over d.

A = IᴬIᴬ or Iᴬi
B = IᴮIᴮ or Iᴮi
AB = IᴬIᴮ
O = ii
Rh+ = DD or Dd; Rh− = dd

P(A+, A−, … O−) = P(ABO) × P(Rh)

ABO and RhD sit on different chromosomes and are inherited independently, so each gene is crossed separately and the two probabilities are multiplied.

Worked example: A (Iᴬi) × B (Iᴮi)

Each parent passes one allele. Parent 1 passes Iᴬ or i; parent 2 passes Iᴮ or i. The four combinations are equally likely (25% each):

Parent 1 \ Parent 2	Iᴮ	i
Iᴬ	IᴬIᴮ (AB)	Iᴬi (A)
i	Iᴮi (B)	ii (O)

So the child has a 25% chance each of AB, A, B, and O. If both parents are also Rh-positive carriers (Dd), the Rh cross gives 75% Rh+ and 25% Rh−, so the chance of an A+ child is 25% × 75% = 18.75% and of an O− child 25% × 25% = 6.25%.

Blood type chart: parents' ABO groups

Possible and impossible ABO groups for the child under standard inheritance (the order of the parents does not matter):

Parents	Possible child groups	Not possible
O × O	O	A, B, AB
O × A	A, O	B, AB
O × B	B, O	A, AB
O × AB	A, B	AB, O
A × A	A, O	B, AB
A × B	A, B, AB, O	none
A × AB	A, B, AB	O
B × B	B, O	A, AB
B × AB	A, B, AB	O
AB × AB	A, B, AB	O

Rh factor chart

Two Rh-negative parents can only have an Rh-negative child. If either parent is Rh-positive, the child may be either, depending on whether the Rh-positive parent carries a hidden d allele (Dd).

Parents	Possible child groups	Not possible
Rh+ × Rh+	Rh+, Rh−	none
Rh+ × Rh−	Rh+, Rh−	none
Rh− × Rh−	Rh−	Rh+

How to use this calculator

Choose each parent's ABO blood group (A, B, AB, or O) and Rh factor (positive or negative).
Read the grid: every child blood type is marked Possible or Not possible.
If you know the genotypes (for example from a family blood-typing history or a genetic test), turn on Advanced and pick each parent's ABO genotype (such as Iᴬi) and Rh genotype (such as Dd).
The advanced mode shows the exact probability of each blood type and the ABO and Rh Punnett squares behind it.

Limitations and medical disclaimer

Prediction only. This tool models standard ABO and Rh inheritance. It cannot tell you your baby's actual blood type — only a blood test can.
No invented percentages. A blood test shows phenotype, not genotype, so without genotypes the calculator shows possibilities rather than population-weighted odds.
Rare variants exist. The Bombay phenotype (hh), cis-AB alleles, and weak or partial D can produce results that look impossible under these rules.
Not a paternity test. Blood type cannot prove or rule out parentage; many unrelated people share a type. Only DNA testing can.
Rh incompatibility in pregnancy. If you are Rh-negative and pregnant, talk to your clinician about antibody screening and anti-D immunoglobulin — it largely prevents haemolytic disease of the newborn.

Tracking health during pregnancy? Try the BMI calculator.

FAQs

How is a baby's blood type determined?: A baby inherits one ABO allele (Iᴬ, Iᴮ, or i) and one Rh allele (D or d) from each parent. Iᴬ and Iᴮ are codominant and both dominant over i, so IᴬIᴬ or Iᴬi is group A, IᴮIᴮ or Iᴮi is group B, IᴬIᴮ is AB, and ii is O. D is dominant, so DD or Dd is Rh-positive and only dd is Rh-negative.
Can two parents with blood type O have a type A or B baby?: Under standard inheritance, no. Type O parents are both ii, so every child is ii (type O). Very rare exceptions, such as the Bombay phenotype or a cis-AB allele, can make blood typing look inconsistent with the parents; these need specialist laboratory testing.
Can two Rh-positive parents have an Rh-negative baby?: Yes. If both parents are Rh-positive carriers (genotype Dd), each child has a 25% chance of inheriting d from both and being Rh-negative (dd). Two Rh-negative parents (dd × dd), by contrast, can only have Rh-negative children.
Why does the calculator show 'possible' instead of percentages?: A blood test reports your phenotype (for example A+), not your genotype. A type A parent could be IᴬIᴬ or Iᴬi, and the odds for the child differ between the two. Instead of guessing with population averages, the default mode shows which child blood types are possible for any matching genotype. Turn on the advanced toggle if you know the exact genotypes to get Punnett-square percentages.
What are the odds if one parent is Iᴬi and the other is Iᴮi?: The ABO Punnett square gives four equally likely outcomes: IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B), and ii (O), so 25% each. If both parents are also Dd for Rh, multiply by 75% Rh-positive or 25% Rh-negative: for example A+ is 25% × 75% = 18.75%.
Can blood type prove or rule out paternity?: No. Blood type can only show that a combination is unlikely under standard genetics, and rare variants break those rules. Many unrelated people share the same blood type. Only DNA testing can establish parentage.
What is Rh incompatibility in pregnancy?: If an Rh-negative mother carries an Rh-positive baby, her immune system can make anti-D antibodies that may affect this or a later pregnancy (haemolytic disease of the newborn). It is largely preventable with anti-D immunoglobulin. If you are Rh-negative and pregnant, discuss testing and prophylaxis with your clinician.

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