In our careful watch to ensure our baby slept on his back to avoid SIDS, our newborn son started to develop a flat head. While only a cosmetic concern, our pediatrician recommended we put him on his side while sleeping to mitigate the potential problem. The challenge was, our active son actually rolled over by himself at 1 week, so was not likely to stay on his side.
We found the Symmetry Sleep Positioner useful to keep him on his side (and rotated sides). In the months before he was able to really move on his own, we place him in the SSP within his crib, and he slept comfortably. The device stopped being useful once he could roll himself off of it, but by then he was moving enough that the flat head syndrome was not an issue.
While we never used the caliper to actually measure change, the improvement was visible.