by Mary Calvagna, MS
After a year of trying, Suzanne, 36, found out she was pregnant with her first child. Her excitement quickly turned to worry when her doctor told her she was Rh negative (Rh-). When a blood test revealed her husband was Rh positive (Rh+), her concern escalated.
What Is Rh? TOP
Rh factor is a protein that can be found on the surface red blood cells. If the Rh factor is present in your blood, your blood type is positive; if it is not present, your blood type is negative. There are four major blood types—O, A, B, and AB. Each of these blood types can be (+) or (-) with respect to Rh.
Genes determine if the Rh factor is positive or negative. You inherit one Rh gene from each of your parents. The combination determines your Rh status, as shown in the table below.
In the United States, more than 85% of people are Rh+. Being Rh- does not affect your health, but it can complicate a pregnancy.
How Does Rh Affect a Pregnancy? TOP
When an Rh- woman conceives a child with an Rh+ man, their child can be Rh- or Rh+. If the baby's blood type matches the mother's (both are Rh-), Rh causes no complications. This may not be the case however, if the baby's blood type is Rh+.
Problems arise when the baby’s Rh+ blood gets into the mother’s Rh- bloodstream by traveling through the placenta. Red blood cells with the Rh factor are foreign to the mother. In an attempt to fight off this foreign invader, her immune system produces antibodies against the Rh+ blood cells. The process is called sensitization—the mother is now sensitized to the Rh factor.
During the first pregnancy, this usually does not present any danger to the fetus. Most often, the baby is born before the mother develops enough Rh antibodies to do harm. Although some red blood cells leak across the placenta during pregnancy, the greatest amount is transferred at delivery. In addition, a miscarriage, abortion, ectopic pregnancy, or amniocentesis can also cause fetal blood to mix with maternal blood.
Rh sensitization can cause problems for a baby in later pregnancies. If a woman who is sensitized is carrying an Rh+ baby, her antibodies to the Rh factor can cross the placenta and attack the baby’s red blood cells. This causes hemolytic disease, or Rh incompatibility. It is an anemia that can cause serious complications in the infant, including brain damage and even death. Although Rh disease is rarely seen in the first pregnancy, later pregnancies are at an increasingly greater risk.
What Can Be Done? TOP
Women who are Rh- but unsensitized are given a shot of Rh immune globulin at around 28 weeks of pregnancy. She is given another shot within 72 hours of giving birth if her baby is Rh+. Rh- women should also be given the immune globulin shot after any event that could cause the blood of an Rh+ fetus to mix with the mother's blood, including:
Rh- immune globulin contains antibodies to the Rh factor. These antibodies seek out the baby's Rh+ blood in the mother's bloodstream, attach to it, and destroy it before the mother can make antibodies of her own. Rh-immune globulin's protective effect lasts for about 12 weeks, so the shot will be given again during future pregnancies.
If a Woman Is Already Sensitized… TOP
An Rh-sensitized pregnancy is a high risk pregnancy. During an Rh-sensitized pregnancy, the mother's blood is tested for Rh antibodies throughout the pregnancy. If the amount of antibodies is rising to an unsafe level, tests are done to assess the health of the fetus. In some situations, the doctor may advise inducing early labor and giving blood transfusions to the newborn child.
In the last two decades, many advances have been made in treating a fetus with severe Rh disease. For example, blood transfusions can be done in utero, using a tiny blood vessel in the fetus's umbilical cord. This highly-specialized procedure is only available in some centers.
Feeling More Assured TOP
After Suzanne's obstetrician explained Rh to her, she felt more at ease. Although she is still a bit concerned, she knows that with careful monitoring backed by the advances of modern medicine, she and her doctor can help keep her baby safe and healthy.
The American Congress of Obstetricians and Gynecologists
March of Dimes
AboutKidsHealth—The Hospital for Sick Children
The Society of Obstetricians and Gynaecologists of Canada
The Rh factor: how it can affect your pregnancy. The American College of Obstetricians and Gynecologists website. Available at:
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Published September 2013. Accessed May 20, 2014.
Rh disease. March of Dimes website. Available at:
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Updated December 2009. Accessed May 20, 2014.
Last reviewed May 2014 by Michael Woods, MD
Last Updated: 5/20/2014
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