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What is a Rhesus Factor?

What is a Rhesus Factor?

Pregnancy
Feb 3, 2018

Many women carry on blissfully through life without knowing their blood type until after the first prenatal visit. And while there are different blood groups, such as type A and type B, there also is a Rhesus factor. That’s the positive or the negative of your blood type. Your doctor or midwife is looking for your ABO compatibility and Rhesus factor and how that could possibly impact your pregnancy. This is important!

Most people are Rh-positive, but if you are Rh-negative, this can have cause some dangerous complications if your significant other has a different Rhesus factor than you. An Rh-positive fetus inside an Rh-negative mother can cause antigens to be produced if any amount of blood ever crosses the placenta during pregnancy. While your and your baby’s blood systems are normally separate, sometimes with amniocentesis, chorionic villus sampling, bleeding from a miscarriage, or blunt trauma to the abdomen can cause mixing to occur. These antibodies, once formed, cause serious issues in future pregnancies because of Rhesus factor sensitization.

Serious problems don’t normally often occur in your first pregnancy because your Rh-positive baby is born before the antibodies can cause anemia in your baby. These antibodies can destroy some of the fetal red blood cells and those cells are what are needed to carry oxygen. If treatment is not offered, however, and you later become pregnant again with another Rh-positive fetus, your future babies are at serious risk of Rh disease. A simple blood test can look for any antibodies to Rh-positive blood. This is called an antibody screen. Your doctor or midwife will usually test your blood during your first trimester and also again at 28 weeks of pregnancy.

To prevent Rh-sensitization you are given a shot in your upper arm of Rh immunoglobulin (RhIg), usually at 28 weeks and, if necessary, within 72 hours after the birth of your baby. The use of RhIg, known as WinRho or RhoGAM, came into use in the 60s and is considered a World Health Organization essential medicine. It is made from donated blood plasma and can help block your immune system from recognizing these Rh-positive antigens. Unfortunately, it is not helpful if you are already sensitized. Your healthcare provider will closely monitor your baby during pregnancy, if this is the case, and it may necessitate a premature delivery or a blood transfusion for your newborn after birth.

Remember, your doctor or midwife will monitor your pregnancy closely, if you are a Rh-negative mother and offer the routine injections as part of your antenatal care. Having a different Rhesus factor than your significant other doesn’t have to be scary…it just needs to be monitored.

We encourage you to learn more about our "What in the World" series. Please see our A to Z index for a whole host of pregnancy, postpartum, and parenting information and terminology.


Loree Siermachesky works as a multi-certified labour and postpartum doula, certified Lamaze childbirth educator, certified breastfeeding counselor, certified placenta encapsulation specialist and a certified car seat technician in Medicine Hat and Lethbridge. She has had the honour of attending over 1400 births in the last 20 years. She is well-known and greatly respected by the medical providers in Medicine Hat, Lethbridge, Brooks, Taber, and Calgary. She cares deeply for this profession and even more for her clientele, honoring them in whatever method of birth they choose, or helping them transition to new parenthood as they wish.