Clinical implications:
Previous studies have shown that massive FMH (>150 mL) is associated with high perinatal mortality—ranging from 33% to 50%—because severe fetal anemia, hydrops, and stillbirth may occur26–29. The diagnosis of FMH is often difficult because it has nonspecific clinical symptoms and is associated with very specific sonographic signs, thus confirmation via laboratory testing such as the KB test or FCM is required 25.
Some have suggested that the KB test should be performed in every woman involved in major trauma during pregnancy to determine the degree of FMH, regardless of Rh status30–33. Studies performed to find an association between FMH diagnosed by positive KB test following trauma and adverse outcome have shown contradictory results17,34–37. Muench and colleagues, based on a study of 71 women, found that the KB test had a 100% sensitivity for the prediction of preterm labor35, hence recommended its use as a predictor of risk for preterm labor after trauma. However, others have not found that the KB test has any predictive value and did not support the routine use of this test as a predictor of adverse outcome34,36,38,39. These reports did however find that the KB test has utility in women who are Rh-negative, in determining the need for additional Rh immune globulin to protect against isoim­munization but has little predictive value of other adverse preg­nancy outcomes such as placental abruption, preterm birth, or fetal hypoxemia.
Interestingly, a recent survey by the College of American Pathologists demonstrated that the KB test is used for Rh positive women in 52% of the laboratories surveyed40. Given its labor intensity and the experience needed to perform the KB test, it is no wonder that some medical centers have started using the FCM. With the use of various known red blood cell group antigens, fluorescent antibodies are used to mark fetal erythrocytes and are quantified electronically. This is a more sensitive test, which may better identify severe FMH cases and predict perinatal outcome.
In our study, even though the vast majority of women involved in trauma during pregnancy had evidence of some FMH, only 11.6% had significant FMH (>30 ml). Our results are consistent with other studies21,24.
As with the results of studies of the KB test, none of the demographic and prenatal risk factors and outcomes we studied seems to correlate with the positive FCM test. The only statistical differences between the study groups was the duration of hospitalization, which probably was affected by the FCM result itself and cannot indicate adverse outcome. In our literature review, we were unable to find studies that examined the relationship between FCM result and adverse outcome.
Despite the contradictory literature regarding the value of the KB test in Rh positive women involved in minor trauma, a recent report demonstrated many laboratories still perform the KB test in this setup40. It is possible that a positive KB test or a positive FCM result by themselves, do not necessarily indicate pathologic fetal-maternal hemorrhage in pregnant women with trauma41. As previously shown, a significant FMH occurred both in trauma and in a low-risk population during pregnancy, and among women with and without third trimester bleeding38,42. Hence diagnosis of placental abruption or fetal distress remains clinical, based on a combination of clinical signs and symptoms.