E. Albert Reece

In the management of an Rh-sensitized pregnant patient, it is important to know the blood types of the father and of the fetus. Unless documented otherwise, fetal blood should be considered Rh-positive. If, after testing, the fetus is Rh-negative, the maternal antibody will not cause hemolytic disease.

If the father is homozygous Rh-positive, the fetus will be Rh-positive, and further fetal surveillance is mandatory. If he is heterozygous Rh-positive, there is a 50% chance that he has passed the D antigen to the fetus and further testing is warranted. If he is Rh-negative, further surveillance is unnecessary.

The most likely zygosity at the Rh(D) locus of a father can be estimated using race, serology, and the Rh-antigen status of his offspring from previous pregnancies. Recent evidence suggests that an Rh(D) deletion results in the majority of Rh(D)-negative phenotypes among Whites.

Increasingly, Doppler ultrasonography is being used to help diagnose severe hemolytic disease in the fetus, particularly of hydrops fetalis, a severe condition in which the fetus develops systemic edema and a build-up of fluid (perfusion) in body cavities such as the pericardium, thorax, and abdomen.

Fig. 17.2 Doppler ultrasonogram of the middle cerebral artery of a fetus

Historically, isoimmunization to the Rh(D) antigen has been the most common and clinically important form of immune sensitization occurring in pregnancy. As a result of the development of RhoGAM, there has been a dramatic decrease in the incidence of Rh isoimmunization among pregnant women in most developed countries.

Even with the substantial decrease in Rh(D) hemolytic disease over the past 40 years, Rh isoimmunization continues to be a problem among pregnant women. This is because some women become sensitive to other blood antigens for which there is no prophylactic treatment.

Maternal antibodies to all non-D antigens are collectively referred to as “irregular antibodies.” Although they may occur naturally as a result of pregnancy, the majority develop as the result of blood transfusions. This type of sensitization occurs because most blood transfusions are compatible only with ABO and Rh(D) antigens.

Many of these irregular antibodies, similar to Rh(D), can produce severe disease in the neonate, including profound fetal anemia and hydrops fetalis. Because many others are likely, maternal antibody screening should at least search for those already demonstrated to cause problems.

Hemolytic diseases caused by sensitization to irregular antibodies now accounts for the largest percentage of isoimmunized pregnancies in the developed world. The presence of irregular antibodies is not related to the mother’s ABO type or Rh status, and it is possible to become sensitized to more than one type of red blood cell antigen.