Both parents B negative
Possible baby blood types: O-, B-. Not possible: O+, A+, A-, B+, AB+, AB-.
| Child | Range | Typical |
|---|---|---|
| Not possible | — | |
| 0% – 25% | 21.9% | |
| Not possible | — | |
| Not possible | — | |
| Not possible | — | |
| 75% – 100% | 78.1% | |
| Not possible | — | |
| Not possible | — |
Range = lowest to highest chance across the genotypes each parent could carry. Typical = weighted by common allele frequencies. Try other inputs in the blood type calculator.
Why these results
A B negative parent carries ABO genes BB or BO and Rh genes dd. A B negative parent carries BB or BO and dd. Each passes one ABO gene and one Rh gene to the child at random.
ABO: every genotype pairing
| Parent 1 | Parent 2 | Child’s ABO group | Share of couples |
|---|---|---|---|
| BB | BB | B 100% | 0.4% |
| BB | BO | B 100% | 6% |
| BO | BB | B 100% | 6% |
| BO | BO | O 25% · B 75% | 87.6% |
Rh factor
| Parent 1 | Parent 2 | Child |
|---|---|---|
| dd | dd | Rh− 100% |
Step through any pairing on the blood type Punnett square.
Pregnancy and Rh
Both parents are Rh negative, so the baby will be Rh negative too and Rh incompatibility does not arise (assuming confirmed paternity).
Can these parents give each other blood?
B negative can receive red cells from O-, B-; B negative can receive from O-, B-. Full rules are on the blood type compatibility chart.
Frequently asked questions
What blood type will a baby of B- and B- parents have?
The child can be O-, B-. Under normal inheritance the child cannot be O+, A+, A-, B+, AB+, AB-. The most likely single type, using typical gene frequencies, is B- (about 78.1%).
Can B- and B- parents have a O+ child?
Not under normal ABO and Rh inheritance. Rare variants (cis-AB, the Bombay phenotype) or a record error are the usual explanations, and blood type never proves or disproves parentage.
Can B- and B- parents have a O- child?
Yes. The chance is 0% – 25%, depending on the hidden genes each parent carries (about 21.9% with typical gene frequencies).
Can B- and B- parents have a B- child?
Yes. The chance is 75% – 100%, depending on the hidden genes each parent carries (about 78.1% with typical gene frequencies).
Can B- and B- parents have a AB+ child?
Not under normal ABO and Rh inheritance. Rare variants (cis-AB, the Bombay phenotype) or a record error are the usual explanations, and blood type never proves or disproves parentage.
Does it matter which parent is the mother?
For the baby’s blood type, no. Each parent contributes one ABO gene and one Rh gene in the same way.