A baby’s blood type is determined by genes inherited from both parents. Each parent passes on one version of the ABO gene and one version of the Rh gene. The combination of those inherited versions decides whether the baby’s blood is A, B, AB, or O, and whether it is positive or negative for the Rh factor.
That is the biology. The practical question — how you actually find out a baby’s blood type — has a different and more surprising answer. In most cases, no one tests a newborn’s blood type at all. Here is what is actually measured, when, and why.
How Is A Babys Blood Type Determined?
Blood type is set at the moment of conception. It is not something that develops or changes afterward. The ABO system and the Rh system are each controlled by a pair of genes — one copy from the mother, one from the father. Whichever versions a baby inherits is what they will have for life.
The ABO gene comes in three common versions: A, B, and O. A and B are co-dominant, meaning if a baby inherits an A from one parent and a B from the other, both are expressed and the baby is type AB. O is recessive. A baby is type O only if they inherit an O from both parents.
Rh works more simply. The RhD gene is either present or absent. If a baby inherits at least one copy of the gene, they are Rh positive. If they inherit none, they are Rh negative. There is no partial Rh in the standard positive/negative sense used in routine care.
Two important clarifications. First, a parent’s blood type does not determine a baby’s blood type on its own — it only narrows the possibilities. Second, knowing both parents’ types still leaves real uncertainty in many pairings. A type A mother and a type B father, for example, can have a child of any of the four ABO types depending on which hidden versions each parent carries.
Why Do Parents Often Not Know Their Newborn’s Blood Type?
Routine newborn care does not include blood typing. This surprises many parents, but it reflects how medicine actually uses the information.
A baby’s blood type matters in two situations: when a baby needs a transfusion, and when there is a concern about Rh incompatibility between mother and baby. Outside those situations, the result would not change any decision. Hospitals do not run tests that will not guide care.
There is also a practical reason. A newborn’s red blood cells are coated with what are called maternal antibodies for the first few months of life. These antibodies are passed across the placenta before birth and can interfere with certain blood-typing methods. A result taken in the first days of life may be less reliable than one taken later. Some labs use techniques that account for this, but it remains a reason clinicians often wait.
If you want to know your baby’s blood type and it was not tested at birth, it can be checked at any time afterward. Many parents simply wait until a routine blood draw is needed for another reason.
What Is the Cord Blood Test and What Does It Measure?
When a newborn’s blood type is checked, the sample usually comes from the umbilical cord right after delivery. This is convenient and avoids sticking the baby. The test looks at two things: the ABO type and the Rh status.
This test is not routine. It is typically ordered when the mother is Rh negative, or when there is another reason to suspect a blood type mismatch. In those cases, knowing the baby’s type helps guide the next steps.
One limitation worth knowing: cord blood can be contaminated with the mother’s blood or with Wharton’s jelly, the substance inside the umbilical cord. That can occasionally produce a result that needs to be confirmed with a sample drawn directly from the baby. If a result seems inconsistent with the parents’ types, a repeat test is reasonable.
What Is Rh Incompatibility and Why Does It Matter?
Rh incompatibility is the main reason a baby’s blood type gets checked at birth. It occurs when an Rh-negative mother carries an Rh-positive baby.
During pregnancy and especially at delivery, some of the baby’s Rh-positive red blood cells can enter the mother’s bloodstream. Her immune system recognizes the RhD protein as foreign and makes antibodies against it. This is called sensitization.
The first pregnancy is usually not affected, because the mother’s immune system needs time to mount a response. The concern is future pregnancies. If she becomes pregnant again with another Rh-positive baby, her existing antibodies can cross the placenta and attack the baby’s red blood cells. This can cause hemolytic disease of the newborn — a condition ranging from mild anemia to serious complications.
This is largely preventable. Rh immunoglobulin (often called RhoGAM) is given to Rh-negative mothers at specific points during pregnancy and after delivery. It works by clearing Rh-positive fetal cells from the mother’s bloodstream before her immune system can respond to them. When given as recommended, it dramatically reduces the risk of sensitization.
This is one area where the evidence is strong and the clinical practice is well established. Rh-negative women should discuss the timing of these injections with their obstetric provider, because the schedule depends on individual circumstances.
Can a Baby’s Blood Type Change?
A baby’s blood type does not change. It is fixed by genetics from conception onward. Once the ABO and Rh genes are set, they stay the same for life.
What can happen is a misleading test result. In the first few months of life, maternal antibodies still circulating in a baby’s blood can cause a blood-typing test to give a wrong reading. This is temporary and does not mean the baby’s actual type changed.
There are also rare genetic variants that affect how blood type is expressed. Some people have weak or partial forms of A or B antigens that can be missed by standard testing. These are uncommon and usually only matter in specific medical situations, such as before a transfusion or during pregnancy. For everyday purposes, a person’s blood type is stable.
How Blood Type Is Inherited: A Quick Look at the Possibilities
Because each parent contributes one ABO version and one Rh version, the possible outcomes depend on what each parent carries. The table below shows the ABO possibilities when both parents’ types are known. It assumes the standard versions of each gene and does not account for rare variants.
| Parent 1 | Parent 2 | Possible ABO Types in Baby |
|---|---|---|
| A | A | A or O |
| A | B | A, B, AB, or O |
| A | AB | A, B, or AB |
| A | O | A or O |
| B | B | B or O |
| B | AB | A, B, or AB |
| B | O | B or O |
| AB | AB | A, B, or AB |
| AB | O | A or B |
| O | O | O only |
For Rh, the logic is simpler. Two Rh-negative parents can only have an Rh-negative baby. If either parent is Rh-positive, the baby may be positive or negative, depending on which version each parent passes on.
This table shows possibilities, not predictions. Even when the odds favor one type, the actual outcome is decided by which specific gene versions the baby inherits — and that is a matter of chance.
When Would a Baby’s Blood Type Actually Be Tested?
A newborn’s blood type is tested in a few specific situations:
- The mother is Rh-negative and the baby’s Rh status needs to be confirmed.
- The baby needs a blood transfusion.
- There is concern about ABO incompatibility causing jaundice.
- A healthcare provider orders it for another clinical reason.
ABO incompatibility is worth a brief mention. It happens when a mother with type O blood carries a baby with type A or B blood. The mother’s anti-A or anti-B antibodies can cross the placenta and break down some of the baby’s red blood cells. This can cause jaundice in the newborn period. It is usually mild and treatable, but it is one reason a provider might check the baby’s type.
Outside these situations, there is no medical need to know a newborn’s blood type. That is not an oversight. It reflects a deliberate approach: test when the result will change what happens next, and skip it when it will not.
If you want to know your child’s blood type for your own records, you can ask your pediatrician to order a test at any well-child visit. It is a simple blood draw and the result is reliable after the first few months of life.
Frequently Asked Questions
How is a baby’s blood type determined?
A baby’s blood type is determined by the versions of the ABO and Rh genes inherited from each parent. One copy comes from the mother and one from the father, and their combination sets the baby’s type for life.
Do hospitals routinely test a newborn’s blood type?
No, most hospitals do not test a newborn’s blood type unless there is a specific medical reason. The most common reason is an Rh-negative mother, so the baby’s Rh status can be confirmed.
Can a baby have a different blood type than both parents?
Yes, this is possible and not unusual. For example, two type A parents can have a type O baby if both carry a hidden O version of the gene.
When can a baby’s blood type be accurately tested?
A blood type can be tested at birth using cord blood, but results in the first few months may be affected by maternal antibodies. Testing after about six months of age gives a more reliable result.
