During labour and delivery (intrapartum transmission)
Transmission during labour and delivery is thought to happen when the baby comes into contact with infected blood and genital secretions from the mother as it passes through the birth canal.
This could happen through ascending infection from the vagina or cervix to the foetal membranes and amniotic fluid, and through absorption in the digestive tract of the baby.
Alternatively, during contractions in labour, maternal-foetal microtransfusion may occur.
Scientists know that transmission occurs during delivery because:
- 50 percent of babies who turn out to be infected test HIV negative in the first few days of life.
- There is a rapid increase in the rate of detection of HIV in babies during the first week of life.
- The way that the virus and the immune system behave in some newborn babies is similar to that of adults when they first become infected.
It is also shown by the success in preventing it happening. This includes:
- Treatments that have reduced transmission risk, even when given only in labour
- Delivering the baby by Caesarean section before labour starts.
If it takes a long time to deliver after the membranes have ruptured (waters breaking) or if there is a long labour, the risk of transmission in women not receiving ARV treatment or prophylaxis is increased.
A premature baby may be at higher risk of HIV transmission than a full term baby.