In the UK, the most common route of infection for children is through mother-to-child transmission, often referred to as vertical transmission. It is also possible for an HIV-positive woman to pass HIV infection to her child through breastfeeding. However, other possible routes such as infection via blood products, organ or tissue donation (Simonds), consensual and non-consensual sex (Robinson) and contamination with non-sterile equipment (Lackritz) have been reported. Infection from non-sterile equipment tends to occur among children from outside the UK who have undergone surgical procedures abroad.

Estimates for the risk of mother-to-child transmission (without interventions) are commonly quoted at around 15-20% in Europe and the USA. In developing countries this figure can be much higher. For example, in some African countries estimates of approximately 35% risk of transmission without interventions have been reported. Breastfeeding accounts for about a third to a half of this difference.

It is believed that approximately 75% of transmission from mother to child occurs around the time of delivery in new breastfed populations, however other factors influence the rate of transmission. The most important of these is the mother’s viral load.

The Aids Clinical Trial Group's (ACTG) 076 trial revealed that the use of AZT during pregnancy reduced the risk of mother-to-child transmission by two thirds. The use of combination antiretroviral therapy can further reduce transmission to one per cent or less in resource-rich settings.

As a result, in the UK since 1999, universal testing for HIV during pregnancy has been recommended by the Department of Health. This has resulted in a marked improvement in antenatal detection rates. It was estimated that in 2004 88% of pregnant women in London and 95% outside London had their HIV infections diagnosed prior to giving birth – a huge improvement from the year 2000, when 82% in Inner London, 65% in Outer London, and 56% in the rest of England and Wales had their infection diagnosed prior to giving birth. The Department of Health set a target of 90% uptake of HIV testing by the end of 2002.

Despite these improvements in detection rates, vertical transmission will remain as the main source of paediatric HIV infection, globally.