What blood type will the baby have if one parent has blood type A and the other B?
If one parent has blood type A and the other B, 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)
- APossibleup to 50%
- BPossibleup to 50%
- ABPossible25%–100%
- OPossibleup to 25%
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 A means either two A copies or one A and a hidden O, and a blood test can't tell which. Blood type B means either two B copies or one B and a hidden O, and a blood test can't tell which. So the baby can have blood type A, B, AB or O.
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 |
|---|---|---|---|---|---|
| A/A | B/B | – | – | 100% | – |
| A/A | B/O | 50% | – | 50% | – |
| A/O | B/B | – | 50% | 50% | – |
| A/O | B/O | 25% | 25% | 25% | 25% |
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 one parent has blood type A and the other B?
With standard inheritance: A (up to 50%), B (up to 50%), AB (25%–100%) or O (up to 25%).
Can parents with blood types A and B have a baby with type O?
Yes, if both parents carry a hidden O copy and both pass it on. The chance is up to 25%.
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.