We would always recommend using our size guide to find the right size for your child, particularly when buying a Happy Nappy™ product. Age ranges are only given as a guide and the measurements should be the main factor when choosing a Happy Nappy™ product. It is best for the baby to be measured whilst laying flat and not immediately after a feed. You should measure all around their tummy, across the belly button, and at the fullest part of their thigh. When worn, you should be able to get a finger comfortably around the edges of the seal and there should be no red marks. Many of our products offer a size guide checker. You can access this by clicking on 'Check my size' on the product page, and entering your child's measurements. Alternatively, the full size guide for all products can be found at the bottom of each page on our website.