Use over regular sleepwear to take the place of loose blankets; 100% polyester micro-fleece * Unzips from the bottom for easy diaper changes; Generous sack design allows room for kicking, but cannot be kicked off so baby stays warm all night; Sleeveless design reduces the risk of overheating * Sleeveless design reduces the risk of overheating * Embroidery reminds caregivers to place baby on his or her back * Size preemie fits birth-5 pounds, size small fits 10-18 pounds, size medium fits 16-24 pounds, size large fits 22-28 pounds, size extra large fits 26-36 pounds