If the diaper is larger, smaller, or has ill-fitting, it will cause leakages and blowouts. The diaper may not tuck firmly around a newborn’s tummy and legs, leaving gaps for the pee to leak out. So, you should get zero size diapers for newborns. On the other hand, if you have a toddler, the diaper might not offer adequate penetrability. So, you should size up or use additional inserts or doublers.