A positive and B negative parents
Possible baby blood types: O+, O-, A+, A-, B+, B-, AB+, AB-.
| Child | Range | Typical |
|---|---|---|
| 0% – 25% | 14.1% | |
| 0% – 12.5% | 5.6% | |
| 0% – 50% | 19.4% | |
| 0% – 25% | 7.8% | |
| 0% – 50% | 16% | |
| 0% – 25% | 6.4% | |
| 12.5% – 100% | 22% | |
| 0% – 50% | 8.8% |
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
An A positive parent carries ABO genes AA or AO and Rh genes DD or 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 |
|---|---|---|---|
| AA | BB | AB 100% | 1% |
| AA | BO | A 50% · AB 50% | 14.9% |
| AO | BB | B 50% · AB 50% | 5.4% |
| AO | BO | O 25% · A 25% · B 25% · AB 25% | 78.7% |
Rh factor
| Parent 1 | Parent 2 | Child |
|---|---|---|
| DD | dd | Rh+ 100% |
| Dd | dd | Rh+ 50% · Rh− 50% |
Step through any pairing on the blood type Punnett square.
Pregnancy and Rh
If the mother is the B- parent, the baby could be Rh positive, so she is usually offered anti-D (RhoGAM) around 28 weeks and again after birth if the baby is Rh positive. If the mother is the Rh-positive parent, no anti-D is needed. See the Rh factor guide.
Can these parents give each other blood?
A positive can receive red cells from O+, O-, A+, A-; 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 A+ and B- parents have?
The child can be O+, O-, A+, A-, B+, B-, AB+, AB-. All eight types are possible. The most likely single type, using typical gene frequencies, is AB+ (about 22%).
Can A+ and B- parents have a O+ child?
Yes. The chance is 0% – 25%, depending on the hidden genes each parent carries (about 14.1% with typical gene frequencies).
Can A+ and B- parents have a O- child?
Yes. The chance is 0% – 12.5%, depending on the hidden genes each parent carries (about 5.6% with typical gene frequencies).
Can A+ and B- parents have a A+ child?
Yes. The chance is 0% – 50%, depending on the hidden genes each parent carries (about 19.4% with typical gene frequencies).
Can A+ and B- parents have a B- child?
Yes. The chance is 0% – 25%, depending on the hidden genes each parent carries (about 6.4% with typical gene frequencies).
Does it matter which parent is the mother?
For the baby’s blood type, no. For pregnancy care, yes: if the mother is the Rh-negative parent and the baby could be Rh positive, she is usually offered anti-D (RhoGAM).