Size 1 fit from 9lbs to 20lbs Size 2 fit from 18lbs to 38lbs Size 3 fit from 30lbs upwards.   We have spent a lot of time over the past 20+ years perfecting the sizes and only rarely do we receive an email saying the customer isn’t happy with the fit. However please remember that babies come in all different shapes and sizes so the above weight and age range should only be taken as an estimate. If you have any questions about size and fit please get in touch care@littlelamb.com Most babies will change shape many times during the early years. It’s quite common for a baby to start using the size 2’s sooner than expected and then to return to fitting the smaller size again once baby has started being mobile and have lost some of their chubbiness. Our OneSize nappy will fit 90% of babies from birth to potty but sometimes a newborn is too small - we suggest waiting until the baby is approaching 9lbs.