Transmission during pregnancy (in utero)
We know that in utero transmission happens because some HIV positive babies, tested when they are a few days old, already have detectable virus in their blood. Usually it takes several weeks after infection until HIV shows in the blood. The rapid progression of HIV in some babies is also used as proof of in utero transmission.
In utero transmission might happen if the placenta is damaged and blood from the mother transfers into the blood circulation of the foetus.
Chorioamnionitis has been linked with damage to the placenta and increased HIV transmission risk.
This might be because HIV-infected cells travel across the placenta. Or it might happen because HIV slowly gets through different layers of the placenta, until the virus gets to the blood that reaches the foetus.
Having a high viral load and a low CD4 make in utero transmission more likely. In utero transmission might also be linked to when women start HIV drugs in pregnancy. It is more likely to happen if women start later in pregnancy and viral load stays higher for longer.Having tuberculosis (TB) at the same time as HIV is also a risk. And HIV also increases the risk of in utero transmission of TB.
Last updated: 1 December 2015.