Is polycotton stretchy?

The key is not so much fabric content as the knit used. Cotton/spandex will work, as we all know--but a woven poplin in poly-cotton is as unyielding as medieval armor. Without a notion of the style you've chosen, Let me make a suggestion. Take a double handful of fabric on the bolt at the store, with your hands about 8 inches apart. Think of the fabric between your hands as a piece of round spider web and visualize the ways in which it must stretch on the body and why. Then stretch your hands apart around opposite sides of a comp circle. This will tell you the directions in which the stretch is best and least. You can even close your eyes if that helps. The one thing they teach badly is a FEEL for the fabric, and this exercise is designed to do that.