A posterior placenta is a placenta that has attached to the back wall of the uterus and is considered to be the normal placement for the placenta. The back wall of the uterus is the side that is closest to the mother's spine, notes eMedicineHealth.
The term "posterior" means in the back and is the opposite of the term "anterior" which means in the front. According to the Estrella Mountain Community College, the position of the placenta depends mostly on where the egg is when it is fertilized. Once the egg is fertilized it then attaches itself to the uterine wall and grows from there.