Rh incompatibility does not produce serious complications in the first pregnancy. Which of the following explains this phenomenon?
Fetomaternal hemorrhage occur only at the time of delivery. Only in 5% cases transplacental transfer of RBC occur in the first trimester. The leak is very small and insufficient to produce primary immune response. Moreover as a primary response IgM antibodies are produced, which due to its large size do not cross the placenta. Therfore primary sensitization occur in less than 1% of first Rh incompatible pregnancy. Ref: Pregnancy at Risk: Current Concepts By Krishna, 4th Edition, Page 387 ; Textbook of Obstetrics By D.C.Dutta, 6th Edition, Pages 333-4
Antibodies are not able to cross placenta
Antibody titer is very low during secondary immune response
IgG generated is ineffective against fetal red cells.
Massive hemolysis is compensated by increased erythropoesis.