There is about a 25-30% chance that an HIV-infected mother will pass on the virus to her child during childbirth (without nevirapine). Why is this not 100%? Don’t they share the same blood supply? I figured that the baby would already have the virus prior to childbirth.
On the second page of the story: “HIV is far more contagious when freshly caught.” What is the reason for this, if true?
For the second part, it most likely has to do with viral load. Early in an infection, there are more virus particles in your blood, making a more concentrated source of infection.
About mother-to-fetus transmission, well, first of all, Mom and baby don’t exactly share the same blood supply. The placanta is a place where mom’s and baby’s blood vessels hang out really close together, allowing a transfer of diffusable things like oxygen, nutrients, and waste. The cells never really meet each other unless there’s a bleed, which is most likely to happen during the birth process. As to why the virus wouldn’t cross over like everything else, I don’t know. Lots of viruses do cross the placenta and can have terrible effects on the baby (rubella comes to mind). Hopefully someone who knows more about HIV can come help us out, because that’s the best I can do.
I can only answer #1: No, mother and fetus do not share the same blood supply. Mothers and babies often have different blood types, which would not be possible if they shared blood.
The placenta has many blood vessels and capillaries in it. Some of them carry the mother’s blood, and some the fetus’s. They come very close to one another, and trade oxygen, nutrients and wastes back and forth through the walls of the capillaries, but the blood itself never touches.
Ideally, anyway. Occasionally, some of the capillaries will tear, and the blood can mix. This can be deadly (in the case of an blood type or rh incompatibility*) or it can mean that blood born diseases can be transmitted.
Mostly, IIRC, HIV transmission happens during childbirth, when the mother tears a bit and bleeds on the baby, but I could be wrong about that part.
*“rh factor” is a protien in the blood. It’s either positive or negative. If the mom is negative and the fetus is positive, and the fetus bleeds while she’s pregnant, her body will start to make antibodies against the fetus’ blood and might kill it. This is usually prevented by medication. In the US, all rh negative mothers are given medication to prevent this in all pregnancies, no matter what they say about the blood type of the father. (They could be wrong, they could be lying. It’s just too risky to chance and too hard to test the fetus directly.)