Arms open to convert from the Original Woombie swaddle sleep sack to an arms-free sleep sack when the time arises to transition the baby. Do what's most comfortable for your baby with either option! Cotton/Spandex blend. Fabric stretchy. The Convertible Woombies have a 92% Cotton 8% Spandex Blend, which results in slightly more restriction, more like the Houdini Woombie.