How likely is mother-to-child transmission of HIV?

Without any sort of treatment or care, the chance of a woman with HIV passing it on to her baby (vertical transmission) is considerable. Before antiretroviral therapy and other preventative measures were introduced, transmission rates per pregnancy were between 14 and 20% in European countries, and up to 43% in African countries. Transmission happened during pregnancy, around the time of labour and delivery, and through breastfeeding.

In 21 countries in sub-Saharan Africa surveyed in 2019, the transmission rate varied between 2 and 25%. The countries with the best figures were Botswana, Eswatini, South Africa and Namibia (all below 5%). Even in countries with good access to HIV treatment for pregnant women, gaps in services meant that the transmission rate is higher: for example, 8% in Zimbabwe and 11% in Kenya. The poorest results were in Nigeria and the Democratic Republic of Congo, where fewer than half of pregnant women received HIV treatment. Across the continent, about half of this transmission occurred during pregnancy or delivery and the other half during breastfeeding.

But with effective HIV treatment and a managed delivery, the risk of vertical transmission is greatly reduced. Transmission from pregnant women with diagnosed HIV has fallen to 0.3% and 0.2% in the UK and France, respectively.

A review of over 100 research studies found that the risk of passing on HIV during pregnancy or birth is zero if the pregnant person:

• started HIV treatment before pregnancy
• had an undetectable viral load (less than 50 copies/ml) when they conceived their baby
• maintained an undetectable viral load until after their baby was born.

Basic information on having a baby

In these specific circumstances, we can say that ‘Undetectable = Untransmittable’ (U=U) applies. For pregnancy, the viral load needs to be below 50 copies/ml for U=U – this is a lower threshold than the 200 copies/ml used for U=U between sexual partners.

The same review of studies found that if viral load is above 50 copies but below 1000 copies during pregnancy, the risk of vertical transmission is around one in 77.

There was also no difference in risk between vaginal and caesarean delivery in people whose viral load was below 1000 copies. 

The researchers also looked at 13 studies of breastfeeding. The risk of transmission each month during breastfeeding was 0.1% if the mother’s most recent viral load was below 50 copies/ml. The risk was 0.3% if their most recent viral load was above 50 copies/ml, and increased to 0.7% if the most recent viral load was above 400 copies/ml.

Those figures are for the risk each month. For people with a viral load below 50 copies/ml, a 0.1% risk per month means that there was still a 1.2% risk of passing on HIV after one year of breastfeeding and a 2.4% risk after two years of breastfeeding.

The researchers noted there was not enough data to confirm that U=U applies to breastfeeding. This is because there are not yet enough studies that have included regular viral load monitoring of people using modern anti-HIV medications while breastfeeding.

If neither mother nor baby are on HIV treatment, there is around a 20% chance of the child acquiring HIV after two years of breastfeeding.

HIV treatment reduces the mother’s viral load so that the baby is exposed to less virus while in the womb, during birth and in breast milk. In addition, some anti-HIV drugs cross the placenta and enter the baby’s body where they can prevent the virus from taking hold.

A number of factors can increase the risk of HIV transmission, including:

• having a high HIV viral load
• starting HIV treatment late in pregnancy
• having a low CD4 count
• HIV-related illness
• a sexually transmitted infection
• bacterial vaginosis.

Women who acquire HIV during pregnancy or breastfeeding are at greater risk of passing on the virus as the viral load is very high soon after infection.

Mothers who have an undetectable viral load are usually recommended to have a vaginal delivery. For women with a high viral load (for example, over 1000), a caesarean delivery lowers the risk of HIV transmission. During childbirth, heavy bleeding and the baby having more contact with mother’s blood increases the risk of transmission, as does inflammation of the placenta (chorioamnionitis).

Giving the newborn baby a short course of anti-HIV drugs, called infant post-exposure prophylaxis (infant PEP), protects the baby. Feeding with formula milk lowers the risk of HIV transmission, but is only recommended in places with access to safe clean water. (If formula feed cannot be prepared with safe water, the risk of HIV is outweighed by the risk of other diseases).

If breastfeeding, exclusive breastfeeding (breast milk only) is safer than mixed feeding (giving other food or formula milk as well). However, it’s recognised that giving breast milk and formula milk may be necessary when establishing breastfeeding and if the feeding parent experiences mastitis or gastroenteritis.

Babies should never be given solids together with breast milk before they are 6 months old as this significantly increases the risk of HIV transmission.