Dear expert,

I have a question pertaining the transmission of HIV from the mother to the fetus. According to what a doctor told me, the placenta won't allow the mother blood to pass into the fetus blood stream at all. only the nutrient from the mother can pass through the placenta. According to the statistic figure, the percentage of transmission from mother to the unborn child during pregnancy is 20-25% if the mother is not on ARV medicine and around 2% if she is. My question is under what condition the mother can transmit the disease to her unborn baby in the womb if the placenta block the crossing of mother infected blood to that of the unborn baby? Why 20-25% of the woman with HIV can pass the virus to her unborn child while the other 75-80% cannot if the two groups are not on ARV?

Thanks for your question. The placenta is a great barrier that protects the developing fetus from HIV. The placenta allows a baby with a different genetic make-up from the mother to grow inside the body without stimulating an immunologic reaction. This is because the blood which carries the cells of the immune system is kept separate. The vast majority of transmission of HIV from mother to child occurs at the time of birth when the membrane is ruptured and the blood systems can mix. That is why on treatment it is most important to have the mother's viral load suppressed at delivery (as low as possible...at least below 1,000 copies) and to give antiretroviral medication during labor and to the baby immediately after birth. This blocks most transmissions. The next most important time for transmssion to the baby is through breast milk. So breast feeding should not be done if there is access to clean water to mix formula (in the developing countries breast feeding is still more safe than bottle feeding with unclean water for the baby's overall health). With these components in place transmission should be well below 2%. If a mother has a very high viral load (especially a mother who becomes infected with HIV while she is pregnant) occasionally the virus can get through the placenta and cause an infection in utero, but this is rare.

As with any exposure, transmission is not 100%. A 20-25% transmission rate is quite high when put in the context of transmssion from sexual contact due to the higher exposure with blood during delivery.

Thanks,

Joe