The last trimester is the worst time for backache! Your increasing size is usually to blame.
I never heard of back labor from your placenta being on the back wall. The baby's position is what causes back labor. If baby is facing up towards your belly ...Then the back of his/her head will press on the hard bones in your back...Causing back labor. However if baby is facing your back....Then the hard part of the head won't be pressing on that.
What causes back labor?
The cervix dilating and effacing and possibly the pressure of the fetus on the tailbone causes back labor. When the uterus contracts in the early stages of labor, the pain may be a radiating pain. This pain may radiate from the front to the back (more common) or from the back to the front. If you had the type of radiating pain from the front to the back, you are more likely to have back labor during the active or transitional phase of labor.