The placenta also performs the important function of protecting the fetus for possible infection - however, it is not always able to distinguish between what is a good substance and what isn't - and this is why pregnant women are asked to avoid substances which may cause harm, such as caffeine, alcohol, herbal substances and drugs. Placenta does not hinder alcohol from entering the system of the fetus.

The alcohol crosses the placenta and reaches the fetus. The amniotic fluid surrounding the fetus will also contain alcohol. Prenatal alcohol exposure puts the fetus at risk for physical and mental damage. Fetal alcohol spectrum disorder (FASD) describes children who have a disorder or disability related to prenatal alcohol exposure. There are many disorders that fall within this spectrum, with fetal alcohol syndrome (FAS) being the most severe.