Your baby’s blood type is determined by the combination of genes inherited from both parents, and it follows predictable patterns. The ABO group (A, B, AB, or O) and the Rh factor (positive or negative) are inherited separately, so you need to consider both. With some basic knowledge of how these genes work, you can narrow down the possibilities, though pinpointing the exact result often isn’t possible until after birth.
How ABO Blood Type Is Inherited
Everyone carries two copies of the ABO gene, one from each parent. The A and B versions of this gene are both dominant, while O is recessive. This means a person with type A blood could be carrying either two A copies (AA) or one A and one O (AO). The same goes for type B: someone could be BB or BO. A person with type O always carries two O copies (OO), because O only shows up when there’s no dominant gene to override it.
This hidden carrier status is why two parents with type A blood can have a baby with type O. If both parents are AO, each has a 50% chance of passing along the O gene. When both pass O, the baby ends up OO, which is type O. The same logic applies to two type B parents who are both BO.
Type AB is straightforward: the person inherited A from one parent and B from the other. Because A and B are codominant (neither overrides the other), both show up. An AB parent will always pass either an A or a B to their child, never an O.
Blood Type Combinations by Parent Pairing
Here are the possible blood types for a baby based on the parents’ types. Keep in mind that when a parent is type A or B, their hidden second gene could be O, which widens the possibilities.
- Both parents type O: Baby will be type O.
- One parent O, one parent A: Baby can be A or O.
- One parent O, one parent B: Baby can be B or O.
- One parent O, one parent AB: Baby can be A or B.
- Both parents type A: Baby can be A or O.
- Both parents type B: Baby can be B or O.
- One parent A, one parent B: Baby can be A, B, AB, or O (the widest range of outcomes).
- One parent A, one parent AB: Baby can be A, B, or AB.
- One parent B, one parent AB: Baby can be A, B, or AB.
- Both parents AB: Baby can be A, B, or AB (never O).
The A-and-B parent pairing is the only combination that can produce all four blood types, because between them they may carry A, B, and O genes.
How Rh Factor Works
The Rh factor (the “positive” or “negative” after your blood type) is controlled by a separate gene. Having the Rh factor means a specific protein sits on the surface of your red blood cells. The gene for this protein is dominant, so you only need one copy to be Rh-positive. About 85% of people are Rh-positive.
Two Rh-positive parents can absolutely have an Rh-negative baby. This happens when both parents carry one dominant copy and one recessive copy of the Rh gene. Each parent has a 50% chance of passing the recessive version, so there’s a 25% chance their child inherits the recessive gene from both sides and ends up Rh-negative. Two Rh-negative parents, on the other hand, will always have an Rh-negative child, since neither carries the dominant gene to pass along.
Why Rh Factor Matters During Pregnancy
If you’re Rh-negative and your baby is Rh-positive, your immune system may treat the baby’s blood cells as foreign and produce antibodies against them. This usually isn’t a problem during a first pregnancy, but those antibodies can persist. In a later pregnancy, they can cross the placenta and attack the red blood cells of an Rh-positive baby, potentially causing a serious form of anemia called Rh disease.
To prevent this, Rh-negative mothers receive an injection around 26 to 28 weeks of pregnancy that stops the immune system from making these antibodies. A second dose is given within 72 hours after delivery if the baby turns out to be Rh-positive. This treatment is highly effective and has made Rh disease rare in countries where it’s routinely used.
Newer noninvasive tests can now determine a baby’s Rh status before birth using a simple blood draw from the mother. These tests analyze fragments of fetal DNA circulating in the mother’s bloodstream, and some regions are beginning to offer them as part of routine prenatal care for Rh-negative pregnancies.
When Results Seem Impossible
Occasionally, a baby’s blood type appears to break the rules. One well-known reason is a rare genetic quirk called the Bombay phenotype. The A and B blood type markers are built on top of a foundation molecule called the H antigen. Nearly everyone produces H antigens, but people with the Bombay phenotype don’t. Without that foundation, A and B markers can’t form, so the person tests as type O even if they carry A or B genes. This is extremely rare but can explain cases where, for example, two type A parents seem to have a type B child.
Lab errors during typing, especially in newborns, can also account for surprising results. A baby’s blood type is sometimes retested later because the antigens on their red blood cells aren’t fully developed at birth, which can occasionally give a weak or ambiguous reading.
How Common Each Blood Type Is
In the U.S., some blood types are far more likely than others. O-positive is the most common at 37.4% of the population, followed by A-positive. On the rarer end, AB-negative occurs in just 0.6% of people, making it the least common type. B-negative (1.5%) and AB-positive (3.4%) are also uncommon. O-negative, the universal donor type often in high demand at blood banks, makes up about 6.6% of the population.
Your baby’s odds of landing on a particular type depend entirely on what genes you and your partner carry, not on population averages. But these numbers can give you a sense of how unusual certain outcomes would be.
When You’ll Find Out for Sure
Most parents learn their baby’s blood type shortly after birth, when a sample from the umbilical cord is tested. This is done routinely when the mother is Rh-negative or has certain antibodies, but it’s not always performed otherwise. If it wasn’t tested at delivery, a simple blood test at any point later in life will give you the answer. Some at-home blood typing kits are also available, though they’re less reliable than lab testing and shouldn’t be used for medical decisions.