Mother-to-child transmission of HIV can occur antepartum, intrapartum or postpartum via breastfeeding. Most cases of mother-to-child transmission occur during labour. As with sexual transmission, R5-tropic viruses are more likely to be transmitted from mother to child. The mechanisms underlying these observations are not completely deﬁned. The overall risk of vertical transmission of HIV is 25-30%. As for sexual transmission, maternal plasma HIV viral load is the predominant risk factor for vertical transmission.  However, HIV transmission can occur despite a low maternal HIV viral load; mothers with plasma HIV RNA < 1000 copies/mL have transmitted HIV to their infants. The prime determinant of transmission, in this context, is absence of maternal ART. Transmission of HIV from mother to baby occurs in 10% of mothers with a low HIV viral load not receiving ART compared with < 1% of mothers on ART, including in Australia. There is an increased risk of transmission, either perinatally or postnatally, when women contract HIV during pregnancy.
Other factors associated with an increased risk of perinatal HIV transmission include low maternal CD4+ T cell count, prolonged rupture of membranes, pre-term labour, chorioamnionitis, cigarette smoking or illicit drug use during pregnancy, and obstetric procedures such as amniocentesis and amnioscopy. Caesarean section before the onset of labour signiﬁcantly reduces the risk of HIV transmission, although ART is the mainstay of HIV prevention and there is no additional beneﬁt of caesarean sections in women receiving effective ART with an undetectable plasma HIV viral load at the time of delivery (see section on Pregnancy in women with HIV infection).
Breastfeeding approximately doubles the risk of mother-to-child transmission of HIV and accounts for approximately one third of cases of mother-to-child transmission.,  High levels of HIV in breast milk cells correlate with increased risk of transmission. Avoidance of breastfeeding has been suggested to reduce the risk of HIV transmission. However, in resource-poor settings, breast milk substitutes may be diﬃcult to obtain and beneﬁts may be off-set by increased respiratory and gastrointestinal infections in non-breastfed infants. Exclusive breastfeeding carries a lower risk of HIV transmission than mixed feeding in South Africa. Other factors which increase the risk of postnatal HIV transmission include maternal nipple lesions, mastitis, low maternal CD4+ T cell count, infant oral thrush and breastfeeding for longer than 15 months.
Variation in the genes encoding some chemokines may inﬂuence the rate of mother-to-child transmission of HIV. For example maternal heterozygosity for the SDF-1-3`A variant of the gene encoding SDF-1a (CXCL12) is associated with almost double the risk of transmission of HIV, independent of maternal plasma HIV viral load. Infant chemokine gene variation has not been demonstrated to inﬂuence HIV acquisition rates.