http://www.aliveandwell.org/html/mothers_babies/mothersbabies.html

At least 75%% of babies born to HIV positive mothers will test HIV
negative without medical intervention. (90) Studies have shown that
for properly nourished HIV positive expectant mothers receiving
regular prenatal care, over 90%% of their children test negative with
no drug therapy. (91) Mainstream medical experts acknowledge that
children need up to 18 months to develop their own immune response and
discard the antibodies passed on to them from their mothers, and note
that HIV testing before 18 months of age does not yield conclusive
results. (92) Despite this widely accepted fact, several states
require mandatory HIV antibody testing for newborns in public
hospitals. (93)