The consequences of Rh incompatibility are not serious during first pregnancy because:
As discussed is the preceeding text – The initial antibodies which are produced are of IgM variety which cannot cross the placenta and by the time IgG antibodies develop, patient has already delivered. Thus first pregnancy is safe.
Another reason is
In sensitised women, anti-D antibodies are produced at such a low level that they are not detected during or after the index pregnancy. Instead, they are identified early in a subsequent pregnancy when rechallenged by another D-positive fetus.
In first pregnancy only IgM antibody is formed
Antibody titer is very low during primary immune response
IgG generated is ineffective against fetal red cells
Massive hemolysis is compensated by increased eryth ropoiesis