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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Antibody Screen Positive: Rh Disease and Other Atypical Antibodies
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9. Rh Disease and Other Atypical Antibodies
Step 9. Management of the At-Risk Fetus
Case Scenario
If the pregnancy is at risk for hemolytic disease, what further testing is indicated?
A. Ultrasound
In addition to maternal titers, follow the fetus with ultrasound. Currently, the best non-invasive test for the early detection of fetal anemia is Doppler of the middle cerebral artery (MCA) to determine the peak systolic velocity. (Detti)(Mari)
The lower the viscosity of the fetal blood as a result of anemia, the higher the cardiac output, and the higher the pulsatility of the cerebral vessels as determined by Doppler. (Divakarin)(Kyle)
Values greater than 1.5 multiples of the median (on gestational age-specific nomograms) have a 98% negative predictive value, a 74% positive predictive value, and a 10% false positive rate, for the detection of fetal anemia. While the data obtained from MCA Doppler is highly informative, the study itself is technically somewhat demanding. The angle of insonation needed to get an accurate value must be zero degrees, or very close to it, so this exam usually requires referral to a specialized center. One can also look for early signs of fetal heart failure (hydrops) with ultrasound. Fetal hydrops is diagnosed when two or more fetal body cavities (pleura, pericardium, peritoneum, or skin) contain excess fluid. Hopefully, the MCA studies will be able to alert you to the development of fetal anemia before it has caused overt high output right heart failure. Use of Doppler ultrasound has decreased the need for invasive testing by over 70 per cent.
B.Fetal blood typing through DNA analysis
With the information above, you may now wish to refer the patient to someone with more experience in perinatal hemolytic disease. (Moise)
If the pregnancy is from a father who is heterozygous for RhD, the fetus will have a 50% chance of inheriting the antigen and will be susceptible to hemolytic disease if the mother has a significant anti-D titer. (Harkness)
While we may soon have available a new test to determine fetal Rh phenotype from fetal cells in maternal peripheral blood, at present it is not yet the standard of care in the United States. Therefore, if you have determined that the fetus is at 50% risk, the next option for the patient is to undergo amniocentesis after 15 weeks gestation to determine the fetal Rh phenotype from amniocytes. If the fetus has not inherited the paternal “big D”, it will be Rh negative, no further testing will be necessary, and the patient can go back to routine prenatal care. If it has inherited RhD, serial titers and ultrasounds, and possibly more amniocenteses, will be needed during the rest of the pregnancy to determine if hemolytic disease is occurring (see below). Likewise, if you know that the father is a homozygote, you know the fetus will have to be Rh positive, and amniocentesis for fetal blood type is not necessary. You will however need follow-up titers every 3-4 weeks to see if increasing maternal sensitization is occurring as a result of the stimulus of occult feto-maternal hemorrhage.
