Blood groups B and AB: what blood type will the baby have?
Your baby can have blood group B, AB or A. Group B is the most likely, at 50%. Group O is not possible.
The Rh factor, positive or negative, is inherited separately: the table further down gives the chances for each Rh combination.
Most likely blood type
B+
Chance of this exact type: 42%.
Blood group
Rh factor
Your baby cannot be
- Group O
Most likely combinations
Hidden genes (such as an O behind an A) are weighted by how common they are in Europe. Rare blood group variants don't follow these rules.
A simplified estimate for fun. Real genetics is more complex.
How we work it out
Each parent passes on one of their two versions of the ABO gene. The square shows which version each parent passes on, how often, and the group the baby gets in each case.
Someone with group B carries a hidden O about 94% of the time, so they can pass on an O.
| Passes on A: 50% | Passes on B: 50% | |
|---|---|---|
| Passes on B: 53% | Group AB: 26% | Group B: 26% |
| Passes on O: 47% | Group A: 24% | Group B: 24% |
And the Rh factor?
Rh positive or negative comes from a different gene, so it does not change the blood group odds above. Two Rh positive parents can still have an Rh negative baby when both carry the negative version.
| Parents' Rh factors | Chance the baby is Rh negative |
|---|---|
| Both Rh positive | 8% |
| One Rh positive, one Rh negative | 29% |
| Both Rh negative | 100% |
If the baby is Rh positive, the pregnant parent's body can make antibodies against the baby's blood, which mostly matters in a later pregnancy. An anti-D injection prevents this. Your midwife or doctor will tell you whether and when you need it.
Frequently asked questions
Can parents with blood groups B and AB have a group A baby?
Yes. With these parents, group A comes up 24% of the time.
Can parents with blood groups B and AB have a group B baby?
Yes. With these parents, group B comes up 50% of the time.
Can parents with blood groups B and AB have a group AB baby?
Yes. With these parents, group AB comes up 26% of the time.
Can parents with blood groups B and AB have a group O baby?
No. These two parents cannot pass on the gene versions that group O needs. Very rare variants, such as the Bombay phenotype or cis-AB, are the only exceptions, and a lab test settles it.