When to switch diaper size newborn to s size?

Actually newborn diapers are very short-lived. I only used them for about a month. I switched when baby's urine output was leaking (wet pants) but her thigh area wasn't tight yet. However when switching from S to M her diaper hadn't started leaking but thigh was already tight. You can also consider to switch based on baby's weight if you're unsure. The guidelines can be seen on the packaging.