If a mother is infected with HIV and is not being given antretroviral prophylaxis:

1. There is a 5% risk that HIV will cross the placenta from a mother to her fetus during pregnancy. The risk is increased further if the woman has an amniocentesis, external cephalic version, becomes infected with HIV during the pregnancy or has AIDS.
2. There is a 15% risk of the infant becoming infected with HIV during labour and a vaginal delivery. The risk is increased if the mother has pre-term labour, chorioamnionitis, rupture of membranes for more than 4 hours, an episiotomy or assisted delivery (vacuum or forceps delivery), a scalp clip for foetal heart rate monitoring or foetal scalp blood sampling for pH. Elective caesarean section removes the risk of HIV transmission during labour and vaginal delivery.
3. There is an additional 15% risk of HIV transmission after delivery if the mother practices mixed breastfeeding (breast milk plus other fluids and foods) for up to 18 months.

Therefore, without antiretroviral prophylaxis, the risk of HIV transmission with vaginal delivery and no breastfeeding is 20% (i.e. 5% plus 15%). With mixed breastfeeding the risk increases to 35% (i.e. 5% plus 15% plus 15%). It is important to know that at least 65% or more of infants born to HIV-infected mothers are not infected with HIV.