For Babies' Short Sleeve Onesies we use American Apparel Infant Baby Rib One-Piece 4001. The neckband has been designed for easy on-and-off. Shrinkage: will shrink an average of one size when put in the dryer. Not intended for sleepwear. If you need more information before choosing your size, please don't hesitate to send us an email at support@birdesignshop.com