Blood types

How is blood type and Rh factor determined?

A person's blood type and Rh factor are genetically determined. A baby may have the blood type and Rh factor of either parent or a combination of both parents. Rh factors follow a common pattern of genetic inheritance. The Rh positive gene is dominant (stronger), which means that when paired with an Rh negative gene, the positive gene takes over.

• If a person has the genes + +, the Rh factor in the blood will be positive.
• If a person has the genes + -, the Rh factor will also be positive.
• If a person has the genes - -, the Rh factor will be negative.

When can problems occur?

Problems can occur when the mother's Rh factor is negative and the baby's is positive, or when a pregnant woman's blood type is different from that of the fetus. In these instances, the mother can form protein called antibodies against the red blood cells of the fetus. In some cases, the antibodies may cross the placenta and attack the red blood cells of the fetus, leading the newborn to have hemolytic disease.