The first major study of HIV prevention in pregnancy was the Paediatric AIDS Clinical Trials PACTG 076 in 1994. This placebo-controlled study demonstrated that the administration of zidovudine to pregnant women and their infants could reduce the risk of perinatal HIV transmission by 67.5% overall.^{[1]0, 11]} More recent studies have explored the efficacy of shorter, less expensive antiretroviral drug regimens, usually more applicable to resource-limited settings, and have also examined regimens to reduce postnatal transmission during labour and breastfeeding.

A number of effective strategies have now been established to prevent mother-to-child transmission during pregnancy, at the time of labour and delivery and in the postnatal period. With the implementation of recommendations for universal prenatal HIV counselling and testing, antiretroviral drug prophylaxis, scheduled caesarean delivery when indicated, and avoidance of breastfeeding, the rate of perinatal transmission of HIV has dramatically diminished to 2% or less in the United States of America, Europe and Australia. In the developed world, the goal of HIV treatment in pregnancy is now to achieve a transmission risk of <1%.^{[1]2 – 15]} A number of factors have been identified that may affect the risk of perinatal transmission. The most significant risk factor is the maternal plasma HIV viral load. Different risks are evident during different times of the pregnancy with labour and delivery being the period of greatest risk for the infant.

There are many factors that may affect perinatal transmission:

1. Viral

• HIV viral load (cell associated/cell free)
• Phenotype (syncytium-inducing (SI), tropism)
• Genotype

2. Immunological

• Decreased CD4+ T cell count
• HIV-specific antibodies (neutralising, ADCC)
• HIV-specific T cell responses
• Mucosal immunity

3. Maternal

• Clinically advanced HIV disease – high HIV viral load
• Primary HIV infection
• Co-infection with hepatitis C virus (HCV)
• Twins – first born

4. Obstetric events ^[16]

• Duration of labour with rupture of membranes (ROM) more than 4 hours in the absence of full virological suppression ^{[17, 18]}
• Use of ART in pregnancy ^[19]
• Mode of delivery – benefit of caesarean section in the absence of full virological suppression ^{[20, 23]}
• Breastfeeding (approximately an additional 30% risk postpartum in the absence of maternal ARVs) ^{[24, 28]}
• Mixed feeding (increased risk over exclusive breastfeeding)
• Some interventions e.g. foetal blood sampling, scalp electrodes

5. Foetal/placental

• Prematurity
• Chorioamnionitis
• Infant immune responses

Significant advances have been made in the prevention of mother-to-child transmission. The proven strategies for prevention are:

1. Use of antiretroviral drugs

• ART for the mother, continuing post-delivery.
• For the baby – antiretroviral post-exposure prophylaxis (PEP). For mothers with HIV viral load <50 copies/mL and prevention of mother-to-child transmission strategies in place, zidovudine monotherapy for 4 weeks is recommended. It is a more complex situation where the risk of mother-to-child transmission is higher e.g. where mothers have not received antenatal ART, and additional ART, eg. zidovudine + lamivudine + raltegravir, for the baby may be considered by the paediatric team ^[20]
• In addition, intravenous (IV) zidovudine may be used during delivery if it is a late diagnosis, or the HIV viral load is >400 copies/mL. ^{[9, 29]}

2. Mode of delivery ^{[21 – 23]}

• The role of caesarean section delivery has changed over time. If maternal HIV viral load is undetectable (<50 copies/mL) and the woman has received ART for more than 4 weeks, there is no evidence of additional benefit from a planned caesarean section delivery and the majority of women are now having a vaginal delivery. ^[12]
• If delivery by caesarean section is planned, it should be undertaken at 38 weeks gestation.
• Avoidance of invasive measures at the time of labour and delivery e.g. scalp electrodes is advisable. The use of forceps and ventouse extraction may potentially increase the risk of transmission but this does not preclude their use if obstetrically necessary.

3. Breastfeeding ^{[24 –27]}

The many benefits of breastfeeding are acknowledged. Researchers have observed transmission through breastfeeding in the range of 0.3% (up to 6 months) and 0.6% (up to 12 months of breastfeeding) where the person was considered to be taking ART during pregnancy and the breastfeeding period. However these studies did not have routine viral load testing results available. Women who desire to breastfeed and who are totally adherent to antiretroviral medication, undergo regular recommended clinical and laboratory monitoring, and have an undetectable HIV RNA viral load (sustained for a given period), now have a choice to breastfeed (exclusive).

It is important for women who choose to breast feed to maintain regular engagement with their healthcare providers and be provided with support for excellent adherence to ARVs. Ideally maternal HIV viral loads should be monitored at four-weekly intervals during breastfeeding to ensure continued viral suppression. Refer current guidelines regarding breast feeding and HIV.

Breastfeeding (exclusive) is still recommended in resource-limited settings without access to a clean water supply.

Women who have questions about breastfeeding or who desire to breastfeed should receive patient-centred evidence-based counselling regarding infant feeding options and harm reduction measures. If women decide not to breast feed, they should have ready access to artificial feeding.