What blood type will the baby have if both parents have blood type O?
If both parents have blood type O, standard inheritance allows the blood types below. The ranges show how the chances shift with gene copies the parents may carry without it showing in their own blood type.
Baby's blood type (ABO)
- ANot possible
- BNot possible
- ABNot possible
- OPossible100%
Based on standard ABO and Rh inheritance. The ranges cover every hidden gene combination. Rare variants can break these rules.
Why these blood types
Blood type O means two O copies, so that parent always passes on O. So the baby can have blood type O. Blood types A, B and AB aren't possible with standard inheritance.
How hidden genes change the chances
A blood test shows each parent's type, not which two gene copies they carry. Each row below is one possible combination, with the baby's chances for that row.
| Parent 1 carries | Parent 2 carries | A | B | AB | O |
|---|---|---|---|---|---|
| O/O | O/O | – | – | – | 100% |
Open the calculator with these parents
And the Rh factor?
The Rh factor is inherited separately from A, B and O. Two Rh-negative parents have an Rh-negative baby; any other pairing can give either, as the table shows.
| Parents | Rh positive | Rh negative |
|---|---|---|
| Rh positive × Rh positive | 75%–100% | up to 25% |
| Rh positive × Rh negative | 50%–100% | up to 50% |
| Rh negative × Rh negative | – | 100% |
Other parent combinations
Frequently asked questions
What blood type will the baby have if both parents have blood type O?
With standard inheritance: O (100%). Blood types A, B and AB aren't possible with standard inheritance.
Can two parents with blood type O have a baby with type B?
No, not with standard inheritance: neither parent has a B copy to pass on. Rare variants such as cis-AB or the Bombay phenotype can break this rule, so a surprising result is worth discussing with a doctor.
Can blood type show who the father is?
No. Blood type only shows whether a combination fits the usual rules, and rare variants or testing errors can make a real family look impossible. Only a DNA test can confirm parentage.