The Rhesus (Rh) factor is a specific protein found on the surface of red blood cells. If this protein is present, the blood type is classified as Rh-positive (Rh+); if absent, it is Rh-negative (Rh-). The Rh factor is one component of an individual’s complete blood type, alongside the ABO group. While Rh status does not affect general health, it is significant during blood transfusions and pregnancy.
How the Rh Factor is Inherited
The Rh factor is determined by a single gene inherited from each parent, following Mendelian genetics. The Rh-positive trait (R) is dominant, and the Rh-negative trait (r) is recessive. A person needs only one copy of the dominant gene (R) to be Rh-positive (genotypes $RR$ or $Rr$). Conversely, to be Rh-negative, a person must inherit two copies of the recessive gene ($rr$), one from each parent.
The Rh-negative trait is recessive and can skip generations, only appearing when two copies of the recessive gene are inherited. This explains how two Rh-positive parents can have an Rh-negative child. If both parents are carriers (genotype $Rr$), they have a 25% chance of passing on the two recessive genes ($rr$) to their child, resulting in an Rh-negative blood type.
Why Rh Status Matters During Pregnancy
Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus. This poses a risk because the mother’s immune system may treat the fetal Rh-positive red blood cells as foreign invaders. Fetal blood cells can enter the mother’s bloodstream during delivery, miscarriage, abortion, or trauma during pregnancy.
This exposure “sensitizes” the mother’s immune system, causing it to produce Rh antibodies (IgG) against the Rh protein. The first Rh-positive baby is usually unaffected because sensitization often occurs late in the pregnancy or during birth. However, once the mother is sensitized, these antibodies can cross the placenta in subsequent pregnancies. If a later fetus is Rh-positive, the maternal antibodies can attack and destroy the baby’s red blood cells, leading to severe anemia and complications known as hemolytic disease of the newborn.
Testing and Preventing Rh Incompatibility
Rh incompatibility is largely preventable through routine testing and treatment. All pregnant individuals are screened early to determine their Rh status. If a mother is Rh-negative, preventative measures are taken to stop her immune system from becoming sensitized.
Prevention involves administering Rh Immunoglobulin (RhIG), commonly known as RhoGAM. This injection contains anti-D antibodies that bind to any Rh-positive fetal red blood cells in the mother’s circulation, clearing them before her immune system can mount a lasting response. RhIG is routinely given to Rh-negative, non-sensitized mothers around 28 weeks of gestation. An additional dose is administered shortly after delivery, typically within 72 hours, if the newborn is confirmed to be Rh-positive.