When to switch diaper size newborn to s size?

It depends very much on the size (weight, build) of baby during birth. For baby above 4kg at birth even can start to s size diaper. Normally after baby turns 1 month old. Some diapers' size such as Merries, the cutting will be slightly bigger and can use until baby is much older. When you feel the baby's thighs are tight with the diaper or urine/poop leaking, it means it's time to switch to diaper which is one sized bigger.

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.

It depends on your baby's birth weight. My baby is borned at 3.34kg. He never used newborn diaper at all. The nurses in hospital started S size when he was new born. However, the rule of thumb is to switch the diaper when you realized that there are frequent leaking of poop and urine.

Newborn diapers don't last very long. The hospital used Huggies Ultra newborn which was meant for babies up to 5kg. Most S sized diapers start from 3kg onwards, so it only takes awhile before your baby is able to wear S sized diapers.

thanks