when and how does placenta develop in human female?

The placenta is an organ found only in mammals during development of foetus. It is made up of cells derived from both mother and foetus. Around 12 weeks of pregnancy placenta (mass of cells) is formed by the cells of both mother and embryo which provides nutrition to the child inside the womb. A tough cord called umbilical cord is also formed during this time which connects the embryo to the uterus through placenta.