O negative and A positive parents
Possible baby blood types: O+, O-, A+, A-. Not possible: B+, B-, AB+, AB-.
| Child | Range | Typical |
|---|---|---|
| 0% – 50% | 30% | |
| 0% – 25% | 12% | |
| 25% – 100% | 41.4% | |
| 0% – 50% | 16.6% | |
| Not possible | — | |
| Not possible | — | |
| 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
An O negative parent carries ABO genes OO and Rh genes dd. An A positive parent carries AA or AO and DD or 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 |
|---|---|---|---|
| OO | AA | A 100% | 15.9% |
| OO | AO | O 50% · A 50% | 84.1% |
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 O- 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.
A group O mother carrying an A or B baby can sometimes develop mild ABO haemolytic disease of the newborn, usually just jaundice that is easily treated.
Can these parents give each other blood?
O negative can receive red cells from O-; A positive can receive from O+, O-, A+, A-. Full rules are on the blood type compatibility chart.
Frequently asked questions
What blood type will a baby of O- and A+ parents have?
The child can be O+, O-, A+, A-. Under normal inheritance the child cannot be B+, B-, AB+, AB-. The most likely single type, using typical gene frequencies, is A+ (about 41.4%).
Can O- and A+ parents have a O+ child?
Yes. The chance is 0% – 50%, depending on the hidden genes each parent carries (about 30% with typical gene frequencies).
Can O- and A+ parents have a O- child?
Yes. The chance is 0% – 25%, depending on the hidden genes each parent carries (about 12% with typical gene frequencies).
Can O- and A+ parents have a A+ child?
Yes. The chance is 25% – 100%, depending on the hidden genes each parent carries (about 41.4% with typical gene frequencies).
Can O- and A+ 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. 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).