A baby’s blood type is determined entirely by the genes inherited from both biological parents. Each parent passes one copy of their ABO gene and one copy of their Rh gene to the child, and the combination of those four gene copies sets the baby’s blood type. Because some versions of these genes are dominant over others, a baby can end up with a blood type that looks different from either parent’s.
How ABO Blood Type Is Inherited
The ABO blood group system has three gene variants (alleles): A, B, and O. You carry two copies, one from each parent, but only certain combinations show up in your actual blood type. A and B are codominant, meaning if you inherit one of each, both are expressed and your blood type is AB. O is recessive: it only shows up as your blood type when you inherit O from both parents.
The A and B genes each produce an enzyme that adds a specific sugar molecule to the surface of red blood cells, creating the A or B antigen. The O gene, by contrast, contains a small deletion that makes its enzyme nonfunctional. It leaves the red blood cell surface unmodified. So a person with type O blood isn’t carrying a different antigen; they’re simply missing both A and B.
This is why two parents who both have type B blood can have a child with type O. If each parent carries one B allele and one hidden O allele (genotype BO), there’s a 25% chance with each pregnancy that the child inherits the O from both sides. The child’s genotype would be OO, giving them type O blood despite neither parent appearing to be type O.
Possible Blood Types by Parent Combination
Because you can’t always tell a person’s full genotype from their blood type alone, the possible outcomes for a baby depend on what hidden alleles the parents carry. Here are the general possibilities based on each parent’s known blood type:
- Both parents type O: Baby will be type O
- One parent type A, one type O: Baby can be A or O
- One parent type B, one type O: Baby can be B or O
- One parent type A, one type B: Baby can be A, B, AB, or O
- Both parents type A: Baby can be A or O
- Both parents type B: Baby can be B or O
- One parent type A, one type AB: Baby can be A, B, or AB
- One parent type AB, one type O: Baby can be A or B (not O, not AB)
- Both parents type AB: Baby can be A, B, or AB
The A-and-B parent combination is the only one that can produce all four blood types. And two type O parents will always have type O children, since neither parent has an A or B allele to pass on.
How Rh Factor Works
The “positive” or “negative” label after your blood type refers to the Rh factor, specifically the D antigen on the surface of red blood cells. Rh positive (Rh+) is dominant, so you only need one copy of the Rh+ gene to test positive. Rh negative (Rh−) is recessive, requiring two copies.
Two Rh-positive parents can have an Rh-negative child if both carry one hidden Rh− allele. In that case, there’s a 25% chance per pregnancy the child inherits the recessive version from both sides. Two Rh-negative parents will always have Rh-negative children, since neither has a dominant allele to pass along.
Why Rh Factor Matters During Pregnancy
When an Rh-negative mother carries an Rh-positive baby, her immune system can recognize the baby’s Rh protein as foreign and produce antibodies against it. This is called Rh incompatibility, and the D antigen is involved in about 95% of cases of hemolytic disease of the newborn. The risk is typically low during a first pregnancy, but once the mother’s body has been sensitized, subsequent pregnancies with Rh-positive babies face increasing danger.
The mother’s antibodies can cross the placenta, especially toward the end of pregnancy, and attack the baby’s red blood cells. This causes fetal anemia and, in severe cases, can lead to serious complications including fluid buildup or stillbirth. To prevent this, Rh-negative mothers who haven’t been sensitized receive an injection of anti-D immunoglobulin around 28 to 30 weeks of gestation, with a second dose within 72 hours after delivery if the newborn tests Rh-positive.
Prenatal blood tests can now determine a baby’s Rh status from a simple blood draw from the mother. Non-invasive prenatal testing for fetal Rh type has shown 100% accuracy in clinical validation studies, allowing doctors to identify at-risk pregnancies early and intervene before problems develop.
When a Baby’s Blood Type Gets Tested
Hospitals don’t routinely test every newborn’s blood type at birth. Testing is typically done when the mother is Rh-negative or has certain antibodies that could cause problems, or when a blood transfusion might be needed. The standard newborn blood spot screening, which checks for metabolic and genetic conditions, is collected between 24 and 48 hours after birth, but this screening isn’t specifically for blood type.
If you simply want to know your baby’s blood type out of curiosity, you may need to ask specifically. Some parents learn it from cord blood testing or from blood work done during a NICU stay. Otherwise, many people don’t learn their blood type until they donate blood or need surgery later in life.
When Blood Type Results Seem Wrong
Occasionally, a baby’s blood type appears to contradict what the parents’ types should allow. Before jumping to conclusions, it helps to know that most parents don’t know their full genotype. A parent who tests as type A could be genotype AA or AO, and these hidden alleles can produce surprises.
In genuinely rare cases, a condition called chimerism can produce confusing blood type results. A chimera carries cells with two different sets of genetic material, which can happen when fraternal twin embryos exchange blood cells through connected placental blood vessels in utero. The result is a person whose blood contains two distinct cell populations, leading to mixed results during standard testing. This is uncommon but well-documented in transfusion medicine.
Even rarer is the Bombay phenotype, where a person genetically carries A or B alleles but lacks a separate gene (FUT1) needed to build the foundation those alleles work on. Without a functional FUT1 gene, the A and B antigens never get assembled on red blood cells, and the person tests as type O even though their actual ABO genotype is something else. People with the Bombay phenotype can only safely receive blood from other individuals with the same phenotype, which makes it medically significant despite its rarity.
Blood Type Distribution Worldwide
Type O is the most common blood type globally, followed by A, then B, with AB being the rarest. The exact proportions vary significantly by population and geographic region. In the United States, roughly 44% of people are type O, 42% are type A, 10% are type B, and 4% are type AB. About 85% of the U.S. population is Rh-positive.
These population frequencies affect the probability of any given baby’s blood type. If you’re trying to predict your baby’s blood type before birth, knowing both parents’ types narrows the possibilities, but the exact outcome depends on the specific alleles each parent carries, which standard blood typing doesn’t fully reveal.