HIV transmission during pregnancy is known to occur because:

• Terminated pre–term foetuses up to 14 weeks old have been found to be HIV-infected.
• PCR tests in the first days of life show that some infants have HIV, suggesting that they were infected in utero.
• Some infants develop HIV–related illness in the first months of life, suggesting that they were already infected in utero. However, this may have more to do with viral load and dose, rather than timing.

Can the risk be reduced?

The chance of HIV transmission increases if:

• the mother has more advanced HIV disease.
• mothers who have already experienced one AIDS defining illness are more likely to transmit HIV to their babies.
• 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 very low or undetectable viral load, which is one of the key reasons for using anti-HIV treatment during pregnancy, it does still occur.
• unprotected sexual intercourse during pregnancy. Women who had unprotected intercourse on at least eighty occasions (an average frequency of once every four days) had a four times greater chance of delivering an HIV–positive baby when all other risk factors (including an HIV–positive partner) were controlled for, according to a study which looked at children born in New York between 1986 and 1994.
• illicit drug use during pregnancy. The biological relationship between illicit 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 (Landesman).
• low birth weight baby. Infants with a birth weight below 2.5 kg (five and half pounds) were almost twice as likely to be HIV–positive when all other factors were controlled for. This effect was seen in low birth weight babies born to mothers who did not use illicit drugs and who did not have AIDS. This association has also been seen in three other studies (Landesman).

It is difficult for studies to estimate the true likely risk of HIV infection from mother to baby if the mother is infected at the time of conception or during pregnancy, since it is difficult to locate and follow a group of just–infected pregnant women. It is possible that the higher rates of mother to baby transmission recorded in Africa are, in part, related to these factors.

HIV–positive women who wish to become pregnant, may be advised to choose to conceive at a time when viral load is likely to be low, or easily controlled by anti-HIV therapy. It should be stressed however, that all HIV-positive women retain the right to make their own choices about fertility and childbirth, regardless of their health, and can expect support from their doctors and health care workers in this regard.