Replacement transfusion for erythroblastosis fetalis

The Rh factor of blood was discovered by Landsteiner and Weiner (1940) and within
a year Levine et al. (1941) that erythroblastosis fetalis was caused by antibodies
produced by Rh-negative mothers against this factor during birth of an Rh-positive
infant, which then caused hemolysis of blood in later-born infants [1, 2].  The initial
technique developed to save lives and protect the health of affected infants was total
replacement of the blood with Rh-negative blood [3, 4].

The question that should be asked is, what causes fetal blood to escape from the
placenta?  One explanation might be that increased pressure in the placenta results
from clamping the umbilical cord before transfer of blood to the infant is complete.  
References
  1. Landsteiner K, Weiner AS (1940) An agglutinable factor in human blood
    recognized by immune sera for rhesus blood. Proc Soc Exp Biol Med. 1940;43:
    223
  2. Levine P et al. (1941) Isoimmunization in pregnancy: its possible bearing on
    the etiology of erythroblastosis fetalis. JAMA. 1941; 116:825–827.
  3. Diamond LK (1948) Replacement Transfusion as a Treatment of
    Erythroblastosis Fetalis. Pediatrics, 1948;2:520-524.
  4. Pearson HA. Commentary: Replacement transfusion as a treatment of
    erythroblastosis fetalis, by Louis K. Diamond, MD, Pediatrics, 1948;2:520-524.
    Pediatrics. 1998 Jul; 102(1 supplement):203-5.