Published: 01 June 2012
  • Transmission can occur when the infant is exposed to infectious blood and body fluids during delivery.
  • An undetectable viral load substantially reduces the risk of transmission; a vaginal delivery can be considered.
  • Decisions about the mode of delivery should be made between the woman, her obstetrician and HIV doctor, based on her particular situation.

A large proportion of mother-to-child transmission events occur during delivery (in medical language, ‘intrapartum’) as the infant passes through the birth canal and is exposed to maternal blood and genital secretions.

Moreover, HIV transmission during delivery is known to occur because tests for HIV’s genetic material (RNA or DNA) on some infants only become positive between 14 and 28 days after birth, suggesting that they were infected at delivery.

In all studies, the most important determinant of HIV transmission is the mother’s viral load (measured in blood around the time of delivery).

For example, a large UK study1 found that in comparison with women who had a plasma viral load below 50 copies/ml at the time of delivery:

  • Mothers with a viral load below 1000 copies/ml had nine times the odds of transmitting HIV.
  • Mothers with a viral load below 10,000 copies/ml had eleven times the odds of transmitting HIV.
  • Mothers with a viral load above 10,000 copies/ml had 49 times the odds of transmitting HIV.

While transmission is rare in mothers with undetectable viral load (below 50 copies/ml), it does occasionally occur.2

Unsurprisingly, women who do not take antiretroviral therapy have a higher risk of transmission. But this factor remains significant even after the impact of viral load has been taken into account.1 Moreover, treatment interruptions during the third trimester have been associated with a greatly increased risk of transmission, perhaps because they can lead to viral rebound near the time of delivery.3

Viral load in genital and cervical secretions is also important, independent of viral load in blood.4 

There are clear links between herpes infection, genital ulcer disease and raised viral loads in genital secretions. Each of these has been associated with increased risks of HIV transmission.4,5

More generally, and as during pregnancy, any infection which causes inflammation of the placenta and membranes surrounding the foetus will increase the risk of HIV transmission.

Prolonged rupture of membranes prior to delivery is a risk factor. In a large meta-analysis, for each extra hour of membrane rupture, the transmission risk increased by 2%. This applied whether the baby was delivered vaginally or by caesarean section.6

When twins are delivered vaginally, the first-born twin spends a longer time in the birth canal than the second twin. This may account for the higher infection rate for first-born twins observed in some studies.7,8

Infants born prematurely have a higher transmission risk. In earlier studies, before effective antiretroviral treatment was widely used, the effect was observed in infants born at less than 37 weeks.9 In the current context, it is only observed in infants born very prematurely, i.e. before 33 weeks.10 Also, infants born with a birth weight below 2.5kg (5.5 pounds) have a higher risk of acquiring HIV.11


  1. Townsend C et al. Low rates of mother-to-child transmission of HIV following effective pregnancy interventions in the United Kingdom and Ireland, 2000-2006. AIDS 22: 973-981, 2008
  2. Tubiana R et al. Factors associated with mother-to-child transmission of HIV-1 despite a maternal viral load <500 copies/ml at delivery: a case-control study nested in the French Perinatal Cohort (EPF-ANRS CO1). Clin Infect Dis 50: 585-96, 2010
  3. De Martino M et al. Is the Interruption of Antiretroviral Treatment During Pregnancy an Additional Major Risk Factor for Mother-to-Child Transmission of HIV Type 1? Clin Infect Dis. 48: 1310-1317, 2009
  4. John GC et al. Correlates of mother-to-child human immunodeficiency virus type 1 (HIV-1) transmission: association with maternal plasma HIV-1 RNA load, genital HIV-1 DNA shedding, and breast infections. J Infect Dis 183: 206-212, 2001
  5. Chen KT et al. Genital herpes simplex virus infection and perinatal transmission of human immunodeficiency virus. Obstet Gynecol.106: 1341-1348, 2005
  6. International Perinatal HIV Group Duration of ruptured membranes and vertical transmission of HIV-1: a meta-analysis from 15 prospective cohort studies. AIDS 15: 357-368, 2001
  7. Duliège AM et al. Birth order, delivery route, and concordance in the transmission of human immunodeficiency virus type 1 from mothers to twins. International Registry of HIV-Exposed Twins. J Pediatr. 126: 625-32, 1995
  8. Biggar RJ et al. The risk of human immunodeficiency virus-1 infection in twin pairs born to infected mothers in Africa. J Infect Dis. 188: 850-5, 2003
  9. European Collaborative Study Maternal viral load and vertical transmission of HIV-1: an important factor but not the only one. AIDS 13: 1377-1385, 1999
  10. Warszawski J et al. Mother-to-child HIV transmission despite antiretroviral therapy in the ANRS French Perinatal Cohort. AIDS 22:289-299, 2008
  11. Birkhead GS et al. Acquiring Human Immunodeficiency Virus During Pregnancy and Mother-to-Child Transmission in New York: 2002-2006. Obstetrics & Gynecology: 115: 1247-1255 , 2010
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.