Ideally, you should wait to begin using the Magic Sleepsuit until your baby is transitioning from the swaddle and fits properly into the Magic Sleepsuit. Proper fit of the Magic Sleepsuit is critical for its safety and effectiveness. When the Magic Sleepsuit fits properly, the scooped neck will lay flat on the baby's chest, away from the face. There should be no fabric near the baby's face or chin. Baby's shoulders should NOT slide out of the neckline, and they should NOT be able to wiggle their arms out of the arm openings. The baby should NOT have enough room in the trunk area to pull their legs into the body of the Sleepsuit. All these examples are signs that the Magic Sleepsuit is too big. The result could be that your baby could wiggle out of the Magic Sleepsuit and there would be something loose in the crib, which is not safe. Also, it will not give your baby the cozy, secure feeling that makes it so effective. The below sizing chart is based on industry standards and these size ranges are guidelines as all babies have different body types which can affect fit. If you have any questions about the sizing of the Magic Sleepsuit, please feel free to send us an email of your baby in the Magic Sleepsuit to email@example.com and will be happy to do a visual sizing assessment.