Published: 01 June 2012
  • A suppressed viral load throughout pregnancy reduces the risk of transmission.
  • Illness during pregnancy raises the risk.
  • Appropriate antenatal care is essential for women with HIV, including sexual health screening and advice on other screening procedures.

HIV transmission during pregnancy (in medical language, ‘in utero’) is known to occur because:

  • Terminated pre-term foetuses up to eight weeks old have been found to be HIV-infected.1
  • When tested within 48 hours of birth, some infants test positive for HIV’s genetic material (DNA or RNA), suggesting that they were infected in utero.

Moreover, HIV can be readily detected in the placentas of HIV-positive women.

The mother’s viral load is the key determinant of whether transmission happens, whether during pregnancy, childbirth or breastfeeding. The most essential preventive measure during pregnancy is the appropriate use of HIV treatment, as described later in this section.

While transmission is most likely to take place in the third trimester (at which point viral suppression is most crucial), high viral loads at any time in pregnancy raise the risk of transmission. A recent French study compared women who transmitted despite having a viral load below 500 copies/ml at the time of delivery with women who did not transmit. A higher proportion of those who transmitted had a viral load above 500 copies/ml at 14, 28 and 32 weeks of pregnancy.2

French data also show that longer periods of antiretroviral use during pregnancy are associated with lower risks of HIV transmission. Transmission can sometimes occur before treatment is begun or before viral suppression is achieved.3 Treatment interruptions during pregnancy – including during the first trimester – have also recently been associated with an increased risk of transmission.4

The mother’s viral load is likely to be exceptionally high if she has recently acquired HIV herself and so has primary HIV infection during her pregnancy. In a study which specifically investigated in utero transmission, mothers with recent infection had more than twice the risk of passing on HIV than other mothers.5 In the period 2002-06, one in seven mother-to-child HIV transmissions in New York State involved a mother who was thought to have acquired HIV during or just before her pregnancy.6

Illness during pregnancy has also been associated with in utero transmission. Explanations for this include temporary rises in HIV viral load, increased immune activation and increased CD4 target cells during illnesses and infections.

More specifically, bacterial vaginosis, sexually transmitted infections and other conditions which cause inflammation of the placenta and membranes surrounding the foetus are associated with an increased risk of HIV transmission.7,8

Malaria can infect the placenta and is associated with increased transmission risk in some, but not all studies.9,10,11

Tuberculosis during pregnancy is also associated with transmission. While tuberculosis has the effect of raising viral load, this does not account for all the increased risk.12

Cytomegalovirus (CMV) is a virus from the family of herpes viruses that many adults have without causing any problems. It may also be transmitted from mother to child during pregnancy or childbirth. Infants who acquire CMV in this way have an increased risk of acquiring HIV too.13

Some – but not all – studies have found that infant girls are somewhat more likely to acquire HIV than infant boys.14, 15, 3, 16 Moreover a meta-analysis, only including infants who were breastfed, found no gender differences.17 One possible explanation is a higher rate of stillbirths and miscarriage for HIV-infected male infants.

The use of recreational or illegal drugs during pregnancy has been linked with an increased transmission risk in a number of American studies. After controlling for other factors, substance use during pregnancy more than doubled the HIV transmission risk.6 However the biological relationship between drug use and mother-to-child transmission is not fully understood.


  1. Lewis SH et al. HIV-1 in trophoblastic and villous Hofbauer cells, and haematological precursors in eight-week fetuses. Lancet 335: 565-8, 1990
  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. Warszawski J et al. Mother-to-child HIV transmission despite antiretroviral therapy in the ANRS French Perinatal Cohort. AIDS 22:289-299, 2008
  4. 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
  5. Taha TE et al. Association of recent HIV infection and in-utero HIV-1 transmission. AIDS 25: 1357-1364, 2011
  6. 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
  7. Wabwire-Mangen F et al. Placental Membrane Inflammation and Risks of Maternal-to-Child Transmission of HIV-1 in Uganda. Journal of Acquired Immune Deficiency Syndromes 22, 1999
  8. Farquhar C et al. Illness during pregnancy and bacterial vaginosis are associated with in utero HIV-1 transmission. AIDS 24(1):153-155, 2010
  9. Brahmbhatt H et al. Association of HIV and malaria with mother-to-child transmission, birth outcomes, and child mortality. J Acquir Immune Defic Syndr 47(4):472-476, 2008
  10. Ayisi JG et al. Maternal malaria and perinatal HIV transmission, Western Kenya. Emerg Infect Dis 10(4): 643-652, 2004
  11. Bulterys PL et al. Placental Malaria and Mother-to-Child Transmission of Human Immunodeficiency Virus-1 in Rural Rwanda. Am J Trop Med Hyg 85:202-206, 2011
  12. Gupta A et al. Maternal tuberculosis: a risk factor for mother-to-child transmission of human immunodeficiency virus. J Infect Dis 203:358-363, 2011
  13. Khamduang W et al. The interrelated transmission of human immunodeficiency virus type 1 and cytomegalovirus during gestation and delivery in the offspring of HIV-infected mothers. JAIDS 58: 188-192, 2011
  14. Thorne C et al. Are girls more at risk of intrauterine-acquired HIV infection than boys? AIDS 18: 344-347, 2004
  15. Taha TE et al. Gender Differences in Perinatal HIV Acquisition Among African Infants. Pediatrics 115: e167-e172, 2005
  16. Galli L et al. Lower Mother-to-Child HIV-1 Transmission in Boys Is Independent of Type of Delivery and Antiretroviral Prophylaxis: The Italian Register for HIV Infection in Children. Journal of Acquired Immune Deficiency Syndromes 40: 479-485, 2005
  17. The Breastfeeding and HIV International Transmission Study Group Late postnatal transmission of HIV-1 in breast-fed children: an individual patient data meta-analysis. J Infect Dis. 189:2154-2166, 2004
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.