If the mother is HIV positive it can be transmitted to the baby, but there are drugs out now that women can take that can block the virus from being transmitted to the fetus and then the baby has to take a medicine for the first six weeks of life. Also they believe that C-section instead of natural child birth can also cut down on the chance of transmission to the baby. Though you don't know for sure if the baby will be positive or negative for sure until the child is 18 months old (a doctor explained to me that this is because an infant has their mother's immune system until they are 18 months old at that point they develop their own immune system). They do test the baby every 3 months up to the test at 18 months. In the case of my daughter her 18 month test was negative and they made me wait another 3 months and then tested her one last time which was also negative. They said that is because the child needs to have two negative tests in a row to be cleared medically of having HIV. So as you can see I've lived this that is why I know all of this.