Size 1 will fit most babies from birth to 9 months. If your baby has a low birth weight the size 1 will not fit until baby approaches 9lbs as the leg elastic will be baggy and containment will be compromised. Size 2 is designed to fit from around 8 months (20lbs) to 2.5 years (38lbs).  Size 3 fits from 30lbs and is designed for toddlers who don’t quite get the hang of potty training, especially good for night time. A lot of babies will be potty trained before they reach this size so will only need the first two sizes.