What factors increase the chance of transmission from mother to baby?

Transmission of any infectious matter (including HIV) depends upon three factors:

• How infectious it is
• How efficient the route of transmission is
• How vulnerable the potential 'recipient' is

Factors which increase the likelihood of transmission of HIV from mother to baby include:

• A high viral load, especially during labour and delivery. A mother with high viral load is more likely to transmit HIV, and transmission is most likely to occur during labour and delivery. However, though transmission is rare in mothers with undetectable viral load, it does still occur.
• A low CD4 count or CD4 percentage.
• Advanced HIV disease or previous AIDS-defining illness.
• Older age of mother.
• Smoking during pregnancy.
• Use of recreational or illegal drugs during pregnancy. The biological relationship between drug use and mother-to-baby transmission is not understood, but a study of 525 women in the US showed that women who used heroin, methadone, crack or cocaine during pregnancy were almost twice as likely to transmit HIV to their babies when all other factors were controlled for.¹
• Frequent unprotected sex, especially with multiple partners, during pregnancy.
• No HIV therapy during pregnancy or delivery.
• Premature delivery.
• Waters breaking more than four hours before delivery.
• Prolonged or difficult labour.
• Cervical or vaginal infection (e.g. bacterial vaginosis).
• Inflammation of the membranes which separate the foetus from the mother at the time of birth (chorioamnionitis).
• A low birth-weight baby. Infants with a birth weight below 2.5 kg (5.5 pounds) were almost twice as likely to be HIV-positive when all other factors were controlled for.¹
• Breastfeeding or mixed feeding (breast or formula).

The mother having another illness, such as TB or malaria, can increase the chance of transmission by elevating viral load or by causing immune activation. Studies have shown that a mother infected with HIV during her pregnancy has a higher chance of then transmitting it to her baby.²

Although mother-to-child transmission has dramatically reduced in the UK and Ireland since the introduction of effective HIV treatment, it still occurs – there were 78 cases in 2007.³ The proportion of these cases is declining in relation to new diagnoses as a whole, but absolute numbers remain relatively stable because of the increasing numbers of HIV-positive women giving birth.

An audit carried out on MTCT cases between 2002 and 2005 found that two-thirds involved babies born to mothers who remained undiagnosed during their pregnancy.⁴ This underlines the importance of antenatal HIV testing in the prevention of mother-to-child transmission.

Other problems included pre-term delivery shortening the time available for women to take antiretroviral treatment and failure to manage poor viral suppression.

Similarly, a study of births to HIV-positive women in New York during approximately the same period found that women who were diagnosed at delivery or afterwards were three times more likely to transmit HIV than others. Women who only attended two or fewer medical appointments during their pregnancy had double the risk of transmitting than other women.⁵