While maternal IgG is actively transported through the placenta into the fetus, IgM and IgA are not; these can thus have no preventive effect. Regarding IgG, neutralizing maternal antibodies might theoretically have a preventive effect, but usually the virus circulating at a given timepoint in an infected person is not neutralized by the contemporary Nabs. The situation is unclear, for a recent review see http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1004283

Maternal antibodies against many endemic infections are known to have protective effect on the fetus (transplacental transfer) and the breastfeeding child (breastmilk transfer), which is the basis for administration of certain vaccines to pregnant women (e.g. tetanus immunization).

In HIV infection this transfer of antibodies does takes place in a similar manner, but HIV infection is peculiar because even though the infection triggers the formation of numerous amounts of immunoglobulins, these are largely non-neutralising antibodies.

Furthermore, the high rate of mutation associated with rapid HIV replication leads to formation of Immunoglobulin antibodies that are only transiently effective, losing their effectiveness soon as newer mutant viral strains appear. It takes some time for the immune system to process new antigens from new viral strains and by the time the immune system completes the process of formation of new antibodies some newer mutant strains of the virus would have emerged to continue replication.

It is also important to recall that breastmilk transmission takes place in spite of the simultaneous presence of maternal immunoglobulins along with HIV particles contained in both the serum and within cellular components of breastmilk.