After amniocentesis, 5–20% of patients have evidence of fetal-maternal hemorrhage as indicated by increases in maternal serum α-fetoprotein (1)(2)(3)(4)(5) or by the Betke–Kleihauer test (6)(7)(8). The Betke–Kleihauer test can differentiate fetal from maternal erythrocytes by the relative resistance of hemoglobin F-containing cells to acid elution, and it is the most popular method of diagnosing and assessing the severity of fetal-maternal hemorrhage (9). The reliability of this test has been questioned, however, because numerous sources of error are associated with it (10). These sources of error possibly contribute to the wide variation in the reported incidence of fetal-maternal hemorrhage; a more accurate method of assessing fetal-maternal hemorrhage is therefore required in the clinical setting of rhesus D-negative pregnant women. The discovery of cell-free fetal DNA in maternal serum and plasma has opened a new avenue for noninvasive prenatal diagnosis and has provided a useful marker of complicated pregnancies (11)(12)(13)(14)(15)(16). The analysis of fetal DNA in maternal serum or plasma has afforded diagnoses of fetal rhesus D status (12) and single-gene disorders (13), as well as the determination of fetal gender (14)(15). Cell-free DNA may be liberated directly from the fetal-placental interface into the maternal circulation (16), or it may be transferred into the maternal …