What is a placenta, meaning where does it grow from? Does it grow from the mother, the baby, or both?

From the fetus. The fertilized egg develops the placenta which implants into the uterus..Usually the back wall.
Baby. The mother provides an intricate uterine lining that is ready to receive & supply the nutrients for the fetus. The genes within the baby's chromosomes provide the directions to the developing embryo on how to create a placenta using these nutrients.Without a proper uterine lining, nothing can develop.If the baby's genes don't have the right information, the conceptus sheds.