The placenta is a generally round or ovular organ with an average diameter of approximately 22 cm. It weighs approximately 450 g and is approximately 2.5 cm thick at the centre of the placental disc. One of its most fundamental roles is to allow exchange of substances in the mother's blood with those in the foetus through different ducts, without ever placing the two in direct contact and therefore providing nutrients and oxygen to the embryo while removing waste (carbon dioxide, urea, etc.) at the same time.
These substances cross the blood-placental membrane which is an effective barrier against some pathogens. On the other hand, drugs and alcohol but also viruses and some parasites can cross from one body to another and cause malformations in some cases (delayed development, mental retardation and physical abnormalities).
Human placenta Credit: Flickr (author Inferis) public domain