Your blood type is typically described by a letter—A, B, AB, or O—but an equally relevant part of this classification is the Rh factor, indicated by a positive or negative sign. The Rhesus, or Rh, factor is a specific protein that exists on the surface of your red blood cells. Its presence or absence has no impact on your general health, but the distinction becomes medically significant when blood types from two different individuals mix, such as in blood transfusions or, most notably, during pregnancy. Knowing your Rh status allows healthcare providers to take proactive steps to prevent potential complications.
Understanding the Rh Factor
The Rh factor refers to the presence or absence of the D antigen protein on your red blood cells. If the D antigen is present, you are considered Rh-positive, which is the status for about 85% of people. If this protein is absent, you are classified as Rh-negative. This status is determined solely by the genes inherited from your parents.
The Rh-positive trait is genetically dominant, meaning a person is Rh-positive if they inherit at least one Rh-positive gene. Conversely, a person is Rh-negative only if they inherit the Rh-negative gene from both biological parents. The absence of the D antigen means an Rh-negative person’s immune system will recognize the D antigen as a foreign substance if exposed to it, which is why Rh status is monitored in certain medical scenarios.
How Your Rh Status is Determined
Determining your Rh status is a straightforward process performed through a common blood typing or “type and screen” test. A small blood sample is drawn, and laboratory technicians test it to see if the D antigen is present. The result is then attached to your ABO blood type, creating the final classification, such as O negative or A positive.
This test is performed routinely throughout a person’s life. It is standard procedure before receiving a blood transfusion or undergoing surgery. It is also a fundamental part of the initial prenatal screening for all pregnant individuals.
The Risk of Rh Incompatibility in Pregnancy
The most significant implication of being Rh-negative is the risk of Rh incompatibility during pregnancy if the fetus is Rh-positive. This scenario occurs when an Rh-negative mother carries an Rh-positive baby, typically when the father is Rh-positive. A problem arises if the baby’s Rh-positive red blood cells enter the mother’s bloodstream. This can happen in small amounts throughout pregnancy, or more commonly during delivery, miscarriage, or abdominal trauma.
If the mother is exposed to the Rh-positive blood, her immune system perceives the D antigen as an invader and begins “sensitization” by producing anti-D antibodies. These antibodies are generally not a concern for the current pregnancy because the immune response takes time to develop. This means the first Rh-positive baby is typically unaffected. Once formed, these anti-D antibodies persist in the mother’s system and are capable of crossing the placenta in a subsequent Rh-positive pregnancy.
When these maternal antibodies cross into the fetal circulation, they attack and destroy the baby’s Rh-positive red blood cells, a condition known as Hemolytic Disease of the Fetus and Newborn (HDFN). The destruction of red blood cells leads to severe anemia in the fetus. This can result in jaundice, fluid buildup, and potentially life-threatening complications. This serious risk is the primary reason why an Rh-negative status requires careful medical management.
Medical Management for Rh-Negative Individuals
Fortunately, the risk of sensitization and HDFN is almost entirely preventable through the use of a medication called Rh immune globulin, often known by the brand name RhoGAM. This injectable product is a preparation of passive anti-D antibodies that are given to the Rh-negative mother. The administered antibodies attach to any fetal Rh-positive red blood cells that may have entered the mother’s circulation. They neutralize them before her own immune system can recognize them and mount a permanent immune response.
To prevent sensitization, the Rh immune globulin is routinely given as a prophylactic injection around the 28th week of pregnancy. An additional dose is administered within 72 hours of delivery if the newborn is confirmed to be Rh-positive. The injection is also necessary following any event that could cause maternal-fetal blood mixing, such as a miscarriage, ectopic pregnancy, amniocentesis, or trauma to the abdomen.