Rubella and CMV IgG indicate past infections and will have no effect on the pregnancy,if the tests were done during the first trimester of pregnancy. But if IgM was positive in the 1st trimester then it has deleterious effects on the baby.A thourough clinical examination and hearing tests etc will establish effects on the baby.Even if the tests( serology) were done in the later trimesters of pregnancy it could still indicate that the mother had contracted infection prior to pregnancy and hence  that there will be no adverse effect on the baby.

Yes, if mothers are positive for rubella or CMV specially in first  three months of baby baby may have many congenital defects like heart defects, cataract in eyes, hearing defect n mental retardation may also occure.

Usually IgG antibodies do not cause infection per se...IgM confirms active disease.

As a response to infection human body produced Antibodies IgG indicates past infevtiom or immunization to the virus or bacteria whereas IgM indicates recent infection..  In reference pregnancy it depends on what time the infection was there during pregnancy.. So requires few more details..