What is the strongest predictor for vertical transmission of HIV?

Viral load is the strongest predictor for vertical transmission of HIV because it directly correlates with the amount of the virus present in a mother's bloodstream. Higher levels of viral load significantly increase the likelihood that the virus can cross the placental barrier during pregnancy, as well as during labor and delivery.

When the viral load is low, the risk of transmission to the infant is considerably reduced. This is why routine assessments of viral load in pregnant women with HIV are crucial, as effective antiretroviral therapy can lower the viral load to undetectable levels, effectively minimizing the risk of transmission.

Other factors like maternal age, geographic location, and body mass index can influence maternal health and access to healthcare but do not have the same direct and significant impact on the likelihood of HIV transmission from mother to child as viral load does.