SNOO sack tucks its special swaddle wings to the sides of the SNOO Smart Sleeper Bassinet to keep them lying flat on their back all night long. Open mesh helps prevent overheating. It is hip safe so it lets you see that the hips are always in the right position. The Large and Medium SNOO Sacks have buttons that allow for the arms to be out of the swaddle. Certified GOTS (Global Organic Textile Standards) Medium Intended for Infants: 12-18 lbs Large Intended for Infants: 18-26 lbs