The Newcastle Sleep Sack is a lightweight sleeveless wearable blanket featuring a downward zipper for easy middle of the night diaper changes. Keep your baby both safe and snug to ensure a peaceful nights sleep. Size Guide: S 0-6 months, 23-28 inches, 10-18 lbs M 6-12 months, 26-31 inches, 15-24 lbs L 12-18 months, 30-34 inches, 22-28 lbs