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