In humans, the placenta (afterbirth) is disc shaped and it is so named because of this discoid shape – the word placenta originates from the Latin root for a flat cake.
During pregnancy, the placenta functions to supply nutrients and oxygen from the mother to the fetus and to remove wastes from the fetus to the mother. Additionally, the placenta produces hormones (chemical substances) which among other actions regulate optimal uptake of nutrients from the mother to the fetus. The placenta also protects the fetus from the mother’s immune system since the fetus is a foreign body and would normally be attacked by the mother’s immune system.
Kliman HJ. From Trophoblast to Human Placenta (from the Encyclopedia of Reproduction). Available http://med.yale.edu/obgyn/kliman/placenta/articles/EOR_Placenta/Trophtoplacenta.html. Accessed 23-03-2011. Go to reference
Human placenta - flat cake