Placenta
However, the placental barrier is not able to protect the fetus from everything that could hurt it.  For example, alcohol goes through the placental barrier into the fetus.  This is why drinking alcohol during pregnancy can cause Fetal Alcohol Syndrome.  The placenta is also unable to filter out many other things.  Chemicals that can cross the placental barrier and hurt the fetus are called teratogens.  Some viruses can also cross this barrier and infect the fetus.  On the good side, many of the mother's antibodies can pass through the placenta, and these can help the baby resist diseases in its first year.