> The mother's blood IS the baby's blood during pregnancy, anything the mother takes the baby gets.
This is not really true[0]:
> In the villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier").
There is some pass-through effect, but it is not 100%, and varies depending on the particular chemical.
[0]: https://en.wikipedia.org/wiki/Placenta#Fetoplacental_circula...