> the placental barrier is probably the weakest such barrier At least, in humans (and other primates and rodents, but not most mammals). Human embryos/fetuses

> the placental barrier is probably the weakest such barrier At least, in humans (and other primates and rodents, but not most mammals). Human embryos/fetuses receive more nourishment and achieve more growth during prenatal development in part because the placenta invades the uterine wall much more—for lack of a better word—parasitically than in other mammalian species. That's great for a lot of developmental stuff for the fetus, but it does make transfer of (say) drugs considerably easier. (It also is the source of a lot of the common maternal complications in human pregnancy and is the reason one should at least be in contact with medical doctors while pregnant, even if a low-intervention or home birth plan is intended....)