After a year of trying, Suzanne, a 36-year-old teacher, 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 panic escalated.

Need for Worry?

In the first pregnancy, "being Rh- and having a husband who is Rh+ is not cause for concern," states Hope Ricciotti, MD, an obstetrician and gynecologist at Beth Israel Deaconess Medical Center in Boston and assistant professor at Harvard Medical School. "However, if the baby inherits the father's Rh factor, it can cause complications in future pregnancies."

What Is Rh?

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 blood types—O, A, B, and AB—and each of these 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.

If you inherit…You will be…
+ +Rh positive
+ –Rh positive
– –Rh negative

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?

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.

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 disease. 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?

"To prevent Rh-sensitization, a shot of Rh-immune globulin is recommended for all Rh negative women at about 28 weeks of pregnancy," Dr. Ricciotti explains. "The shot is also recommended within 72 hours after birth, miscarriage, abortion, ectopic pregnancy, or amniocentesis."

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…

Certain situations, such as an improper blood transfusion with Rh+ blood, can cause an Rh- woman to be sensitized to Rh factor. "An Rh- sensitized pregnancy is a high-risk pregnancy and needs to be followed by a qualified obstetrician or high-risk specialist," Dr. Ricciotti cautions. "That said, not all sensitized pregnancies result in poor outcomes."

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 to assess the health of the fetus are conducted. In some situations, the doctor may recommend 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

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 in this area, she and her doctor can help keep her baby safe and healthy.