Please see our sizing chart available on our product page that will provide an approximate guideline to choose an appropriate size for your baby. The Magic Sleepsuit is available in Small 3-6 months (12-18 lbs.), and Large 6-9 months (18-21 lbs.). Babies are all different sizes and shapes so the weight ranges are just guidelines. It is not recommended to size up. Please understand that this can compromise the safety of the baby with the risk of the baby slipping down inside the suit, or being able to wiggle out of it if too big. Additionally, the Magic Sleepsuit will not do what it is intended to do if it does not fit properly. For hip health, the Magic Sleepsuit should fit such that your baby is able to externally rotate hips/legs (feet turned out). Proper fit is critical to the safety and effectiveness of the Magic Sleepsuit. Please reach out to us with any sizing questions, and feel free to send a picture of your baby in the Magic Sleepsuit for a visual sizing assessment.