A person’s blood type is determined by the presence or absence of specific proteins, called antigens, on the surface of their red blood cells, categorized by the ABO system and the Rh factor. While parents may be eager to know this genetic detail, the blood type of a newborn is not routinely tested as part of standard postnatal care. The determination of a baby’s blood type usually occurs only when there is a specific medical need.
Routine Testing Versus Medical Necessity
In the majority of healthy newborns, a full ABO and Rh blood typing test is not performed before leaving the hospital. The standard newborn screening, often called the heel stick test, screens for metabolic and genetic disorders but does not include general blood typing. The decision to test is driven entirely by immediate clinical concerns that require this specific information for the baby’s care.
One of the most common reasons for immediate blood typing is the development of severe jaundice, which is a yellowing of the skin and eyes. Jaundice is caused by high levels of bilirubin, a substance produced when red blood cells break down. If the baby’s blood type is incompatible with the mother’s—such as an O-type mother and an A-type baby—the mother’s antibodies can cross the placenta and cause accelerated red blood cell destruction in the infant, leading to elevated bilirubin that may require intervention.
Another immediate medical necessity for blood typing is the potential need for a blood transfusion. If a baby is premature or suffering from severe anemia and requires a blood product transfusion, the medical team must know the baby’s exact blood type to select compatible donor blood. A blood sample, often taken from the umbilical cord at birth or a heel stick afterward, is sent for immediate analysis. This testing ensures the baby receives a compatible transfusion, preventing a potentially fatal immune reaction.
The Critical Role of Rh Factor Incompatibility
The mother’s Rh status is the primary factor that dictates early newborn blood testing. The Rh factor refers to the D antigen on red blood cells; a person is Rh-positive if they have it and Rh-negative if they do not. If a mother is Rh-negative and the baby is Rh-positive, this creates a condition known as Rh incompatibility, which can lead to Hemolytic Disease of the Newborn (HDN).
During pregnancy, a small amount of the baby’s blood can enter the mother’s circulation, often during delivery. If the mother is Rh-negative and exposed to Rh-positive red blood cells from the baby, her immune system may recognize the Rh factor as foreign and begin producing antibodies against it. These antibodies usually do not affect the first pregnancy, but they remain in the mother’s system and can attack the red blood cells of a subsequent Rh-positive fetus.
To prevent this sensitization, an Rh-negative mother is routinely given an injection of Rh immune globulin (RhoGAM) around 28 weeks of pregnancy. The baby’s Rh status must be confirmed at birth to determine if the mother needs a second preventative dose. If the newborn is found to be Rh-positive, the mother receives the second injection within 72 hours of delivery to clear any fetal cells from her bloodstream. If the baby is Rh-negative, the mother does not require the postnatal shot.
How Blood Type is Determined (Basic Genetics)
A baby’s blood type is inherited from the parents through a combination of genes, following predictable patterns. The ABO system involves three possible alleles: A, B, and O, where a child inherits one from each parent. The A and B alleles are dominant over the O allele, meaning a person with one A and one O allele will have Type A blood.
If a person inherits both an A and a B allele, they will have Type AB blood, as both are expressed equally. The only way to have Type O blood is by inheriting the O allele from both parents. For example, two parents with Type A blood can still have a Type O child if they both carry the recessive O allele.
The Rh factor is inherited separately, determined by a single gene with a dominant and recessive form. The Rh-positive trait is dominant, meaning a baby only needs to inherit one positive gene from either parent to be Rh-positive. To be Rh-negative, the baby must inherit the recessive negative gene from both the mother and the father. While knowing the parents’ blood types can give a high probability of the baby’s type, only laboratory testing provides absolute confirmation.