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David Barrere, MD, FACOG

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My wife is pregnant and just found out that her blood is Rh negative. What does this mean. what are the risks for her and the baby? - Hunter in Peoria, Illinois

The expert answers:
Hunter, having Rh negative blood means that a protein, the D antigen, is not present on the surface of the red blood cells. This condition occurs in about 15 percent of Caucasians of European ancestry, 5 of African-Americans, and about 1% in Asians.

During pregnancy, there is a risk that baby's blood and mom's blood could mix. If mom was Rh negative and baby was Rh positive, then the possibility of a problem exists. This is called Rh isoimmunization. The D antigen on baby's blood can be recognized as a foreign entity by mom's immune system. She can generate antibodies that can cross the placenta and attack baby's blood cells, resulting in anemia.

At the first obstetrical visit, routine blood tests are performed. A few of which are the blood type, Rh factor, and antibody screen. If a patient is Rh negative, then a medication called Rhogam is administered both at 28 weeks and again after delivery, if the child is Rh positive. Giving Rhogam in this fashion decreases the chances of isoimmunization to less than 1%. Rhogam is an antibody that can bind to the D antigen. Rhogam does not cross the placenta, so it only affects baby's blood cells that have transferred into mom's circulation. This prevents mom's immune system from having the opportunity to recognize the antigen and create a response of its own. Rhogam is also administered to Rh negative mothers who undergo certain procedures such as amniocentesis, chorionic villus sampling, and external version. If vaginal bleeding or trauma occurs during the pregnancy, Rhogam can be considered.

Quite often, a woman becomes sensitized to the D antigen prior to pregnancy. This most commonly results from a blood transfusion that was not properly matched. Platelet transfusions carry a higher risk for this.

If a mother were to become sensitized to the D antigen, then monthly titers of her antibody response must be measured. When a level called the "critical titer" is reached, then the possibility of fetal anemia exists. Below this titer, the risk to the fetus is small. At this point, amniocentesis is utilized to look for evidence of blood cell breakdown (hemolysis), and these values are plotted on a chart called the Liley Chart. Depending on the severity of hemolysis, either a plan consisting of 1) close observation with a repeat amniocentesis in 2 to 3 weeks or 2) transfusion can be made. A procedure called percutaneous umbilical blood sampling (PUBS) is now utilized to directly assess the baby's blood count and also to transfuse blood if necessary. The need for this procedure is very rare as isoimmunization is not common.PregnancyAndBaby.com

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