An “RHD detected” result on a blood test indicates that a person has an Rh-positive blood type. This determination is a standard component of blood typing procedures, which become particularly relevant during prenatal care. Finding out that your blood is Rh-positive is very common and simply provides a more complete picture of your health profile. This information helps medical professionals offer the most appropriate care throughout pregnancy.
The Science of Rh Factor
The Rhesus (Rh) factor is an inherited protein found on the surface of red blood cells. If your red blood cells have this protein, you are Rh-positive; if they lack it, you are Rh-negative. The “D” in RHD refers to the most significant antigen in the Rh blood group system. The presence of the D antigen is what a standard blood test looks for to classify your blood type as positive.
Your Rh status is determined by the genes you inherit from your parents. Because it is a genetic trait, being Rh-positive is a natural variation, much like having a specific eye or hair color. It is not a disorder or illness. The majority of the global population is Rh-positive, with about 85% of people in the UK having this blood characteristic.
Understanding Rh Incompatibility
Rh incompatibility arises specifically in pregnancy under a precise set of circumstances. It occurs when a mother with Rh-negative blood is carrying a fetus with Rh-positive blood, a trait inherited from the father. If the father is also Rh-negative, the baby will be as well, and no incompatibility will exist. The issue is not with the mother’s or baby’s blood type itself, but the mixture of the two different types.
During pregnancy or, more commonly, during delivery, a small amount of the baby’s Rh-positive blood can cross the placenta and enter the mother’s bloodstream. Because the mother’s immune system does not recognize the Rh protein, it perceives these fetal red blood cells as foreign. In response, her body begins to produce antibodies to identify and destroy these Rh-positive cells. This initial immune response is known as sensitization.
This sensitization process rarely poses a danger to the first pregnancy because the antibodies are not created in significant amounts until after the baby is born. The risk emerges in subsequent pregnancies if the mother carries another Rh-positive child. Her prepared immune system can quickly produce a large volume of antibodies that cross the placenta and attack the developing fetus’s red blood cells.
Preventative Measures and Monitoring
Fortunately, Rh sensitization is almost always preventable. The primary method of prevention is an injection of Rh immunoglobulin (RhIg), often known by its brand name, RhoGAM. This injection contains ready-made antibodies to the Rh factor. These prepared antibodies work by neutralizing any Rh-positive fetal blood cells in the mother’s circulation before her own immune system can react and create its own permanent defense.
For an Rh-negative mother, Rh immunoglobulin is administered at specific points during pregnancy. An injection is given around the 28th week of gestation as a preventative measure. A second dose is administered within 72 hours after delivery if the newborn is confirmed to be Rh-positive. RhIg is also given after any event that could cause fetal and maternal blood to mix, such as an amniocentesis, a miscarriage, or abdominal trauma.
In addition to preventative injections, monitoring is part of standard care. An antibody screen is a blood test performed on the Rh-negative mother to check if she has already developed antibodies against Rh-positive blood. For mothers who are already sensitized, this monitoring becomes more frequent to track the level of antibodies and assess any potential risk to the fetus. Non-invasive prenatal testing (NIPT) can also be used to determine the fetus’s Rh status from a maternal blood sample early in pregnancy.
Potential Effects on the Newborn
If Rh sensitization occurs and is not managed, the mother’s antibodies can cause a condition in the newborn called Hemolytic Disease of the Fetus and Newborn (HDFN). The term “hemolytic” refers to the breakdown of red blood cells. The maternal antibodies target and destroy the baby’s Rh-positive red blood cells, leading to several health issues for the infant.
The primary consequence of HDFN is anemia, as the baby’s red blood cell count drops faster than it can be replenished. This destruction of red blood cells also releases a yellow substance called bilirubin. A newborn’s liver is not mature enough to process large amounts of bilirubin, causing it to build up in the blood and lead to jaundice, a yellowing of the skin and eyes.
Treatments for HDFN are effective. For mild jaundice, phototherapy is used, where the baby is placed under special blue lights that help break down the bilirubin in the skin. In more severe cases of anemia, a blood transfusion may be necessary to replace the baby’s red blood cells. Due to the success of routine screening and preventative Rh immunoglobulin treatments, severe cases of HDFN are now uncommon.