Stretchy wraps are generally made of knits such as jersey or interlock. It is easy to take babies in and out of a stretchy wrap. This can be easier for the wearer as the sling often remains tied on and the baby is lifted out and put back in as required. Several factors influence stretchiness: carriers with any spandex or lycra content will tend to be very stretchy, carriers which are 100% cotton or other natural fibers will tend to have less lengthwise stretch.
Two way stretchy wraps stretch along their width and length and can be a little easier to get the tension right as they are completely tied before little one is put in the sling. In the library these are the ones that we tend to send out for newborn hires.
One way stretchy wraps only stretch along their width, these are partially tied and then little one is placed in and then tightened up around little one to ge them secure, they start to work in a manner similar way to a woven wrap,
All different manufacturers of stretchy wrap have a different amount of stretch, thickness and length.