These reusable waterproof swim diapers are lined with a comfortable, soft mesh liner, and have an adjustable waist, tied to ensure a snug but comfortable fit. Unlike regular diapers, the swim diapers are designed to contain solid waste, which is mandatory for public pool safety. Paired with the reusable swim diaper, our complementing baby sun hats offer all-day sun protection. S - 0-6 Months. M - 6-12 Months. L - 12-24 Months.