Appropriately sized sleep sacks are safe for a baby to use as soon as they’re born. However, because of the startle reflex, many newborns like to be (and sleep better) swaddled, so you will likely want to consider a swaddle sleep sack that has flaps with Velcro, which can be wrapped around the baby’s arms. If your baby starts to break out of the swaddle or obviously wants their arms free, many of these sleep sacks allow you to wrap the flaps around their torso, so their arms are free. But they’re still getting the feeling of light compression to help soothe them.